The best way to tackle such a question philosophically is start with an extreme case that evinces the point you want to make, and then lessen the extremity until your argument is demonstrated at a more reasonable level.
Let's start with the following extreme situation to show that all humans are potential child-abusers. If you were compelled under duress to choose between A) Abusing a child yourself for 10 minutes or B) Witnessing a million people abuse a million children for 30 minutes then torturing them with red hot pokers and killing them, I'm pretty sure under those conditions that just about everyone would increase their chances of choosing to abuse a child for 10 minutes.
That extreme scenario demonstrates that everyone 'could' be a child-abuser under 'some' conditions. But clearly that situation is not an ordinary circumstance - and mostly we like to consider only what we feel to be in the realms of the norm and the realistic. This is fair enough, except for one potential problem; life is full of vagaries, and there are plenty of potential emotional tipping points dotted about, not just in extreme scenarios, but in everyday life too, making it very hard to say what is realistic in any ‘one size fits all’ model. The problem is, when asking what we humans are capable of, we find the question is the wrong question to ask if it is left in isolation from context-dependent behaviour-altering scenarios. Even the mildest humans can be incited to lose their temper or become violent when pushed too far - like, say, if they felt threatened, or saw harm being done to their family (we've all seen how people change beyond recognition when confronted with a burglar in their house) - and similarly, if any of us found ourselves fighting for our lives in a Middle Eastern civil war or an African genocide, then I don't think it's unreasonable to say that all humans are capable of murder, despite all having different tipping points.
Moral capability leads to circularity
Yet although this paints a clear picture of how life can get its teeth into our varying psychological and emotional states with the slings and arrows of human experience, the importance of the discussion is usually based on the challenge of having it framed in terms of what is realistic, and what is probable for the majority of people under fairly standard day-to-day conditions. The trouble is, that tends to lead us into circular reasoning. Circular reasoning says that A is true because B is true; and B is true because A is true. When trying to assess what is probable for the majority of people, we find that most humans are not realistically capable of unspeakable acts, but what belongs within the realms of the realistic is contingent on the varying duress and psychological pressures that life throws up, which immediately changes what is realistic. In the above consideration we are seeing that John is capable of duress-induced murder only because duress induced him to murder. That is the circularity - most of us are not capable of unspeakable acts so long as we do not find ourselves in conditions that engender the need for unspeakable acts, which doesn’t really tell us much other than that humans are capable of what humans are capable of.
Moreover, even without the above circularity it is hard to pin down moral feelings and moral behaviour to simple considerations, because there are two ways that these things are changing all the time. One is that a perceived immoral act can be committed due to a change in feeling about whether that act is moral or not; and the other is that a perceived immoral act can be committed due to a change in the person committing the act. In early Roman times a young man might not have felt much guilt about wanting to marry and have sex with a 12 year old girl, whereas nowadays he'd be accused of being a sex-fiend. That's a change of perception about the act itself - it was once widely permissible, it no longer is. On the other hand, a young mother (let's call her Jenny) might be a long way from killing her boyfriend in the general day to day sense, but one day if she caught him trying to rape her daughter she might bludgeon him to death with a rolling pin. In this case that's a change in the person's state of psychological duress with regard to her capability of committing murder or manslaughter, not a change of view in the permissibility of murder with a rolling pin. We might have more sympathy with a mother who killed her daughter's attacker with a rolling pin than someone who did it for fun, but that's an issue of mitigation and leniency; it is not a change of view about death-by-rolling pin.
What we're saying here is that if one day in the distant future humans evolve a culture in which murdering sexual offenders is endorsed then feelings about the wrongness of murdering a would-be rapist with a rolling pin would diminish (that is, change in feeling about whether an act is moral or not). We are also saying that Jenny is a potential murderer, but that potentiality only turns to actuality when she is pushed beyond a limit far in excess of her ordinary day to day life (that is, change in the person committing the act).
John Milton, in his Areopagitica, brilliantly sums up the notion that we are all potential murderers, thieves and violent people waiting to happen, whereby we could all find ourselves in situations that test us or change us beyond what we can imagine. He says:
I cannot praise a fugitive and cloistered virtue, unexercised and unbreathed, that never sallies out and sees her adversary, but slinks out of the race where that immortal garland is to be run for.”
You may be more familiar with this wisdom in its other appearance as the maxim 'Virtue untested is no virtue at all'. What is being implied by Milton is something quite acutely perceptive – it is fairly easy for most people to behave quite decently when they are under no pressure, or when fear of the law curbs their instincts for misbehaving – but it is much harder, and much more commendable, when a man or woman faces the toughest challenges yet still comes away exhibiting goodness and kindness. I'm with Milton here; without the illusion that we are really quite decent, without the thin 'social contract' fibril keeping us just about united in our aims, and without ever being subjected to external pressures strong enough to significantly change our behaviour for the worst, I think we would be doing many more bad things than we currently do. Moreover, we know from the psychological experiments of Zimbardo and Millgram how reprehensible people can be when put in conditions that allow them freedom to treat others beyond what they would do in normal society.
And taking it even further, when things are highly extreme they can end up changing history forever. For example, Adolf Hitler, who, by comparison to any decent standard, was stupid in extremity, and highly unmindful of the qualities that knit humanity together, was able to dominate a great intelligent nation like the Germans. The main influence he seemed to wield as a control mechanism was that of absolute certainty (manifested with the likes of Himmler, Goering and Goebbels in things like Lebensborn, National Socialism and the SS temples). Absolutism combined with power brought a sense of pseudo-Wagnerian Germanic certainty that had the power to predominate a generation of people, who, in many cases probably would have been ordinary citizens in another life.
Even Martin Heidegger - one of the best philosophers of the 20th century - was a supporter of Adolf Hitler, which goes to show what the thrall of authority and disconsolation can do even to highly intelligent minds. Heidegger had plenty to say about original freedom with regard to the extreme opposites of chaos and law, and the subjection of chaos to a form to a particular "mastery which enables the primal wilderness of chaos and the primordiality of law to advance under the same yoke" - which, as we now know with hindsight was a very virulent National Socialism under Hitler to which he was attracted for a time. Nazi Germany proved to be, among many things, a nasty national social experiment that produced an animalistic brutality, cruelty, and eradication of human life - a kind of nationwide Millgram test where obedience took primacy over human rights and moral accountability.
Nazi Germany may be an anomalous example, but it shows us the kind of extremes we're talking about when situations take a dramatic turn - be they in war, religious groups or extreme politics. As much as it upsets the sensibilities of those who like the extreme moral propositions of good and evil as polar opposites, and those who view people as off-the-peg typology fodder that sit rigidly on that spectrum, these binary simplicities merely portray a skewed interpretation of what humankind is capable of when she "sallies out and sees her adversary". There were officers in Nazi concentration camps who were good husbands, fathers, and friends - and nobody who knew them under those conditions could have conceived of what they got up to as a result of obedience to authority. Similarly, there have been many seemingly dedicated, kind, committed husbands, wives, boyfriends and girlfriends who have shocked those around them (and themselves) by acting in ways they wouldn’t have thought possible had they not sallied out and seen their adversaries in the shape of temptation and external pressures.
We see occurrences of this character variance in everyday life too. I know a man who, for most of the week, is a placid and inoffensive kind of chap, but who on match day puts on his tribal outfit (otherwise known as a football shirt) and turns into (by his own admission) an abuse-chanting, sanguinary gang member. He tells me that on match day he is "fuelled by hatred and aggression - which feels great, and is a real adrenaline rush".
I notice too how this effect occurs in the difference between being a pedestrian on the street and being a driver in the car. Most drivers know this too. When you accidently step in someone's way in the street or in a shopping mall (or they you), usually you are both apologetic as you try to be as unobtrusive as possible. But when someone cuts you up in a car (which in most cases is accidental, or due to low confidence behind the wheel or poor concentration) our adrenaline levels rise and we become agitated, and, in the case of some, the horn is beeped aggressively.
These are less extreme versions of our application of Milton
So I would say the extent to which we are capable of gross and heinous things depends on a few factors - the psychological duress, the emotional instability, the fear, the vulnerability, the extremities of the situation, the ability to control a desire, the power afforded to us, our perception of what we can get away with, and maybe even the extent to which we are readily able to dehumanise others.
Important: the question in reverse
Here's another thing to consider. We ask whether we are all capable of those heinous crimes for which we love to vilify others - but the question should also be asked in reverse; could those 'despicable' people that we love to vilify actually have been ordinary, morally decent citizens if they'd have been born under different circumstances? I think the answer is yes, they could. I remember seeing a documentary about notorious child killer Myra Hindley's background prior to her meeting fellow killer Ian Brady. The psychologist who looked extensively at her upbringing, and the life choices she made, said (quite rightly in my view) that regarding many other alternative conditions under which she didn't meet Ian Brady she would have led a fairly normal, uncontroversial life. I feel pretty sure that the same can be said for the 20th century monsters often proffered as the epitome of evil - Hitler, Stalin, Mao and Pol Pot - if they had been taken from the cot when they were babies and given to loving parents in rural Staffordshire they probably would have grown up to be pretty decent fellows, leading fairly ordinary lives. Equally, there are, no doubt, many people living in the UK right now who would have caused a death toll as high as Hitler, Stalin, Mao or Pol Pot had they have had any dictatorial power in a country full of people able to be exploited and dehumanised.
As much as all the above is true, I would suggest that in spite our evolutionary legacy, and the fact that we are all potential unspeakable criminals given the wrong circumstances, I think in all probability we are 'not' all realistically (and I do stress 'realistically') capable of unspeakable crimes, where 'realistic' means having a reasonable probability of avoiding the kind of situations in which these things become conducive. That is to say, most of us do have the ability to resist wickedness and depravity, even though I think it lurks dormantly in us all. The big challenge for those who can resist doing despicable things is, as ever, in facing up to how they react to people who haven't been able to. I suppose given the acknowledgement that any of us could be involved in despicable things in the wrong kind of circumstances, we have seen ample reason to prefer kindness, understanding, mercy, love and grace, and not choose to dehumanise those who are not in the same boat as us.
Internal moral inconsistency
The other thing to consider is that humans are morally inconsistent. I remember hearing about a local traveller in my area who frequently beat his wife and had several affairs too. One day when his son smoked some dope he went mad and accused him of putting a slur on the family name. To most people, the wife-beating and the infidelity constitute worse immoralities than smoking a bit of dope - but clearly not in this man's case. I think we see this frequently in human beings - due to upbringing, mental ability and experiences, we find that just about everyone has a sense of right and good over wrong and bad, but they often differ in the particular values they hold dear or the strength of feeling regarding varying matters. We find people who strike us as generally unkind and uncaring, but who adopt a work ethic we find admirable. There are men who make wholly honest shopkeepers but wholly dishonest husbands. Even bloodthirsty tyrants will scoff at discourtesy or bad manners when in formal capacities, or get sentimental about twee things that would have no effect on us.
Our values and ethics seem to be dependent on upbringing and personal experiences, and given that these things differ from person to person, it is not surprising that values and ethics differ too. Consider too that humans are idiosyncratic, and that history is full of things that were considered morally good at the time that we now consider to be morally bad. There are principally two ways that a human will differ from you in values and ethics - one is that they come from another culture, and the other is that they come from another era. A woman from England in 1066 might be as culturally and ethically different from a present day Englishman as a present day woman from Tanzania or Sri Lanka
Of all the people who have ever lived, each one has a particular vision of how our world 'ought' to work, and many others both from different cultures and different historical eras find those 'oughts' quite absurd or inconsistent. But here's the real shocker; given the gross acts we've seen in history, it's a perturbing thought that just about everyone is striving for some kind of goodness or template or ideal concerning how they think things would be best. It seems to me that very few people actually do bad for bad's sake - even the twisted visions like Hitler's ideas about Aryan white supremacy and Mao Zedong's 'leap' towards modernisation and industrialisation amount to a deranged goal for their version of a better kind of world, with scant regard to the lives considered expendable. No doubt in cases like Stalin's quasi-Marxist/Leninist ideology and Saddam Hussein's Ba'athist methods for controlling Iraq, their tyranny is more about taking advantage of power and being corrupted by it - but I think if we focus on the general man on the street, I would say most are striving for some kind of goodness, it's just the case that personal circumstances condition the success or failure of that goal. Everyone has an idea about justice - but views differ on what justice is in the cases of the particular.
I think this is the interesting distinction that puts human progression on a knife edge; on the one hand we have a shared desire for progression, but one that contains differences of opinion regarding the particulars. And yet on the other hand people's life circumstances really are unpredictable, which means that we cannot be complacent in forming pictures of people. Are most of us capable of atrocious things? You bet we are - it's just that thankfully most people never find themselves in the kind of situations in which they could do their worst.
