Heroes: Usually only fights for one creature and will carelessly waste the lives of millions of other creatures just to save that one creature and nothing else.
Villains: Cares deeply about life and creatures other than their own, but unfortunately doesn't like that one creature, so they have to be treated as the villains and often used to tell horrific morals.
When I play Half-life 2 I sometimes get bored and find ingenious ways of "accidentally" killing the friendly NPCs by punting objects around with the gravity gun so that they fall on the NPCs and kill them (it's not supposed to be possible to kill them, but I have my ways.) I like to kill Alyx. I don't hate her or anything, but it's funny.
Playing Oblivion I take matters more seriously and get incredibly guilty when I have to torture or kill a defenceless NPC. I've been an assassin as part of the Dark Brotherhood, but it felt wrong and against my nature. I loved the Thieves Guild, though, which makes me probably chaotic good. I've flat out killed people who I just don't like, though, usually powerful and influential people who want to harm the innocent.
i liked the "fuck everyone else" option fallout new vegas offered since it actually felt like i was building my own narrative as opposed to following someone else's under the label of good guys and bad guys. what if i don't want to be a tool? what if i don't want to be told what to do for someone else's benefit all the time?
generally, if it's at all possible, i like to play games in a morally dynamic way, doing and saying what i really want and not because someone told me to, and not according to a boring linear script. which is why i enjoy games with flexible moral compasses and freedom of action, because they truly allow you to do just about whatever you want, and doesn't force you to be a pawn just because it's in the narrative to be "good" or "evil"
and, well, you know, i just love toying with the emotionless selfish piece of shit "i hate everybody so they dont matter to me unless they get in my way" head-canon.