I've just written an entire book on cruelty. One of its arguments is that we're wrong to think of cruelty as a third-person noun like 'traffic' -- something only other people do.
Refusing to realise that we too can be cruel leads to the kind of situations described by David Frankfurter in his book Evil Incarnate (recommended): hideous atrocities done to people accused of hideous atrocities, in the name of justified punishment or pre-emptive self-defence.
In Cruelty, I write about the processes of 'otherization' which can push people into increasingly cruel behaviour. Processes like the diffusion of responsibility, justifying one's own behaviour, recruiting support from others, and so on.
And yet, knowing all this, I find myself involved in just such otherization! Happily handing over as much responsibility as I can to other people, recruiting the neighbours, and justifying my behaviour by complaining about the harm my enemies have done.
Me! I'm a well-educated, bien-pensant, white, British post-Christian (Steven Rose said so in the Guardian newspaper, so it must be true). I should know better. I do know better! But I do it anyway. How potent is this otherization thing? (And what on earth is a post-Christian anyway?)
My victims aren't human, of course. I live in a terraced house, and the whole row's recently been plagued by rats. They're living two doors down and use my back yard as a through route to my neighbours' garden, which has lots of nesting birds. Fewer now, since the rats started eating the eggs. My neighbours love birds.

I don't mind rats myself, but I share the house with a convinced musophobe (and bird-admirer) and the terrace with several more. So we called the local authorities. They've put down poison, and we'll wait and see.
I feel so bad about this! If only I could talk to the rats and tell them to skedaddle, fast, and steer clear of our houses in future. If only they'd realise they're not wanted (it's been made pretty clear) and do the decent thing by moving on.
Yet despite my qualms I've agreed to the poison. That's otherization for you. I even find myself doing it deliberately! Rehearsing all the reasons why rats are dangerous and undesirable, and telling myself that if we hadn't made that call a neighbour would have (which is certainly true).
A facile example? Perhaps. Certainly killing rats isn't like killing people (though some unsavoury types in our not-too-distant past have claimed that killing certain kinds of people is like killing rats). But many of the psychological mechanisms which lead people into atrocities aren't specific to those extreme situations. They have their roots in much more ordinary human behaviour.
Of course we view cruelty to humans as morally worse than cruelty to rats (well, most of us do). All I'm saying is, similar processes may operate in both cases.