I am always a nice guy, except when I'm not allowed to be or if it's for a quest. See, if there is a quest that requires me to go whack somebody, and it isn't one of those deals where this is a clearly evil quest branch that terminates other possible avenues by eliminating quest givers, than I will always take it - my goal is to do everything I possibly can, provided it isn't the evil option in a binary choice. If being evil for a while means a whole bunch of additional quests that I'd never see otherwise and which don't interfere with the good guy type quests, then that's what I'll do.
Take Fallout 2: In the city of New Reno, there are several crime families you can do various jobs for and eventually become a member of - I would work for all of them. See, if you turn down 3 of the 4 crime bosses, those organizations turn on you and try to kill you, and since wiping out other crime families is a quest, I can almost join the one, turn them down, kill them all guilt free since I'm being attacked, go to the next boss, accept the reward for wiping out that family, turn him down, wipe that family out, and then finally join crime family #3, who are comparatively not very threatening or hostile. Family #4 I also kill, but that's because the head of the family catches me sleeping with his wife (which I totally have to do no matter how I might feel about it otherwise simply because you can get stat upgrades) and possibly also his daughter, and then tries to make me kill people of the "good guy quest giver" variety, so when I refuse they attack me and I kill everyone except his wife and daughter - because I'm a good guy, whee!
But I am never evil simply because I can be - I will steal everything that isn't nailed down because I remain cognizant at some level that NPCs don't really need their stuff and I'm the only character with true personal agency, but I won't insult them or kill them simply because I have the option to do so, and I will happily give away vast fortunes or refuse rewards if given the option to do so.
Essentially, I'm a nice guy unless meta-gaming gets in the way, which it does from time to time (hence why I will do stuff only to turn around and sell people out as that way lies more XP, heh).