I honestly think it's tied to random things. I didn't kill a lot of people out of the blue, mostly just a natural dialogue then choosing the evil answers.
Sounds like it's specific, dialogue-driven triggers rather than generic misbehaviour that gets a person that tag, then.
Occasionally I try to RP a really unpleasant character but to date I've never managed to make myself do it for long. I just feel a bit rotten if I do.
