Well, I kill coyote puppies in F:NV and I feel like a monster afterwards, so it's strange.
That being said, I play as a sniper, so if it wasn't for the little red ticks saying who was going to attack me if they saw me, I'd be murdering people based solely on their dress code.
... I felt disturbed by this revelation.
Of course I try to minimize casualties in any game I play, so here's where I draw the line:
If they're attacking me/ will attack me, need to die to advance the game, or have done something that disgusts me, I consider it open season.