I don't feel any guilt about randomly and psychopathically murdering nearby NPC's, usually because I only do that when I'm bored and feel like victimising the nearest object for kicks. But when I'm playing seriously (as opposed to butchering cute-eyed puppies for the hell of it) I feel genuinely guilty when taking the dark path and will even feel bad about letting allies die in battle when I'm fighting beside them. (when I'm not the one murdering said allies.)
As a result, heroes under my command tend to have two distinct personalities...
Serious mode: All sweetness and light, this hero wouldn't hurt an ant, even if the cost of not hurting said ant was cold-blooded torture for the hero.
Psycho mode: This hero kills things for the lulz. Good or evil... or just minding its own business by the roadside. He/she won't go out of his/her way to torment you psychologically, much preferring the path of full-blown physical annihilation of all who dares to share the same space as him/her.
An even better summary is that I'm either neutral good or chaotic evil in games. NPC's pray that I'm taking my heroic duties seriously when I stroll into town.