Some people really are evil. Some psychopaths, for example, know that what they're doing is negative and do it anyway. (Many more cover it up and try to do the right thing, of course.)

But of course this doesn't apply to terrorists or most criminals. The occasional serial killer fits the bill and not much more.

What really gets me is that if you start taking about terrorists like this, saying you should understand what makes them tick, that they think they're doing the right thing, etc., people freak out. I just think we should understand them better so we know how to stop them, but any attempt at understanding is taken as justification for what they do.