They might even brag about what they're doing as if it's something to be proud of - like Nicholas Prosper, who wanted to commit the biggest school massacre ever (and luckily was stopped in his tracks).
They just don't see their victims as human beings but as worthless objects to be "dealt with" in the name of some sort of personal grudge or grievance or festering hostility. Something has obviously gone horribly wrong in their upbringing.
I couldn't possibly injure another person, let alone kill them. I couldn't possibly do something with such horrendous consequences both for me and for the other person - no to mention for their family and friends. How could I possibly justify my action? Likewise I couldn't bring myself to commit suicide.
As for those individuals who're happy to start wars and kill thousands or even millions of people - I can't imagine how they manage to sleep at night, how they're able to continue with their duties as if nothing unusual is happening.
It just stupefies me. The mental state of someone who can do such things is so far removed from my own mental state that the gap is unbridgeable. I can't even begin to understand what's going through their mind. Can anyone?
One thing it makes very clear - you can never know what's going on in someone else's head. Maybe something wonderful. Or maybe something utterly hideous.