I can't at all understand what drives someone to do something purely evil. The only reason I can think of is insanity.
Insanity is defined as "in a state of mind that prevents normal perception, behavior, or social interaction". Something has to be messed up in their head to think "hey I should shoot a bunch of people in a movie theater".
For terrorists who are carrying out attacks against perceived enemies, I kind of get it. I mean, this is how US gained independence... We threw tea into the harbor, we fought with terrorist-like warfare (back then you lined up and shot at each other, you didn't sneak around and pick people off like we did). We wouldn't have won if we didn't do all that.
Now, we have these places where terrorists are coming from. Some probably think they're fighting for freedom. These people are suppressed and displaced. They live in crummy conditions, are wildly under-educated, and they blame the West for the state of their nation/culture (maybe rightfully so, I'm not so sure), or blame us for interfering with how they want their culture to be.
These people are almost certainly being manipulated. Through their religious beliefs, through direct threats, through whatever... But there are leaders and the leaders believe they're doing something "right". Either they too believe they're fighting for a cause, or they're insane.
What I don't get are some of their targets. To me, they're targets picked by an insane person, someone out for revenge without having a target for that revenge. There's little logical reason, except to invoke fear, but invoking fear itself isn't going to get them anywhere. They're insane targets... there's no reason to it. There's nothing rational about it. It's the work of insanity.