What I want to know is how people deal with the cognitive dissonance of their (presumed) conviction that they're doing good, in the context of the methods that they're employing?
Some of the correspondence of the Nazis has been published, and some of it touches on this. If memory serves, it went something like "doing the right thing is hard, murdering people is hard, therefore murdering people must be the right thing to do." Yes, seriously.
Isn't there ever a moment of "Holy shit, my quest to make the world a better, more natural place is now manifest in me doing things like shooting nuns and throwing acid in infants' faces. I think I'd better go back to my hometown and spend a few weeks crying hysterically in the shower."
Admitting that you have a problem takes guts. It's hard enough when the worst you've done is puke into a gutter; imagine what it would take to admit that throwing acid on someone's face was actually a horrible thing, not a courageous act of religious or ideological commitment. Add the fact that hatred and violence are addictive, and it should hardly be surprising that people who've given in to them avoid admitting this to the last - and if they do admit it, they make up some bullshit story about being unable to change, as opposed to simply unwilling, thus turning themselves into the real victims, at least in their own minds. Which then justifies further degenerate acts in the name of vengeance.
Wouldn't you rather enjoy the high of self-rightenousness and adrenaline than face the hangover?