Adam Morton argues that we use the term 'evil' to refer to actions which we are inhibited from modelling. We often create a mental model of the future. That model might include an immoral action. I might think to myself 'if I couldn't pay the mortgage, would I forge bank notes?' - and I can model that. I bet you can too, you can think about, and rule it out. It's stupid, it's illegal, it may even be immoral, but it's not evil.
But consider this model: 'If I couldn't pay the morgage, would I torture an old lady until she gave me all her money?' You can read the words, and you know what they mean, but you can't (easily) model it. You can't really consider it as a course of action you might take (without making a big emotional effort, whch is quite unpleasant - try it, it feels horrible). That's because it's evil.
Still not with me? I bet when you thought about forging bank notes you thought things like 'well, you'd need special equipment and skills which I don't have', but I bet you never thought that about torturing the old lady. You'd have to force yourself to get over the inhibition even to think a tiny bit about what would be involved in practical terms. It's almost impossible. And thank god for that.
In the book Morton talks about some of the ways in which people overcome the inhibition against evil: psychopathology, fear, conformity, brutalisation, and I think most significant - lack of imagination.
Anyway, all this speculation occasioned by torture in the news. Also by the problem of writing, because writers often have to model evil actions. Morton suggests that we often make evil inhuman (as in Buffy) to help us to overcome the inhibition against modelling it.