Mick (via whom) gives an appropriate short answer - 'speak for yourself' - to this piece by Brian Masters. Masters is claiming that anyone could turn into a murderer like Tomohiro Kato. A longer answer might point, first, to the fallacy that enables Masters to reach this conclusion, and, second, to the fact that the very evidence he appeals to will confute what he says.
Masters moves from 'predisposition to violence is an inherently human characteristic' to 'in the wrong circumstances, each of us could have been him [Kato]' without any intermediate argumentative steps. But the truth of the premise doesn't suffice for the conclusion. There may well be such a human predisposition to violence, but this doesn't show that some people won't resist it, even under any circumstances, at least in the sense that they won't be willing to slaughter other people.
And that, indeed, is what the evidence points to - the very evidence to which Masters himself appeals. For, predictably, he makes use of the 'ordinary men'-type evidence, which shows, from the Nazi period, that the killers and their accomplices and helpers in the years of German-led genocide were drawn from a cross-section of the relevant populations: they were not abnormal monsters (except in so far as they made themselves monstrous in a moral sense); they were just people. All this is well-established. But in the evidence that has established it there are also - virtually always - the counter-examples. There are those who stood out, who did not and who would not. This is so in the case of Christopher Browning's Ordinary Men. Most of the members of Reserve Police Battalion 101, when called upon to shoot down unarmed men, women and children, did so. But some would not. They were a small number, but some all the same. In the notorious Milgram experiment, some subjects would not administer the (simulated) electric shocks as instructed to. Living in the hell of the concentration and death camps, where behaving according to any normal moral code became all but impossible if one wanted to survive even a few days, many of the inmates tried to preserve some residue of morality nonetheless.
It is a libel upon humankind to say that everyone is a potential murderer. And to tell people that, deep down, they are all as bad as they can be risks encouraging them to be worse than they are.