In my opinion, determining whether someone's a bad person is pretty easy. Do they hurt other people in a unnecessary and malevolent way?

This question encompasses emotional, psychological and physical senses of 'hurt': only the last one really brings you under legal liability, but the damage done under the rest is just as painful. If you're only hurting yourself, that's stupid and a problem, but hardly wrong in a moral sense. Sometimes, you have to injure other people - self-defense is justification sometimes. And some people honestly can't tell when they're hurting someone - think "Of Mice and Men". I've never understood the moralistic concern with sex and drugs, although it's clear people can be hurt through their abuse.