In Fallout 2 I tried poisoning Vault City's mayor, then I had to exterminate the whole town. In the same game in New Reno I was caught after banging Bishop's wife and daughter and had to kill almost all the "family". I didn't really repent, the first were a bunch of hypocrites, the second gangsters, but I had to reload and undo the first thing to avoid breaking some quests, while I reloaded in New Reno, but just to try again to bang Bishop women until I managed to avoid being caught. 
Bishop's daughter called my bluff anyway and bore a grudge against me for the rest of the game. Besides getting pregnant. 
In Thief III I stole also the money left from the Abysmal Gale's captain to his widow to reach 100% loot.
In Gothic I unnecessarily killed some human foes after defeating them, to collect more XP.
In Planescape I revealed Silent King's secret to Acaste, triggering a bloody ghoul riot, but after enjoying the massacre, I reloaded to undo it, to avoid having skeletons and zombies hostile.
Normally, anyway, in RPG's I try to keep a good alignment, although I choose or reach (if dynamic like in Planescape: Torment) lawful too only if the best weapon I like in the game requires it, otherwise I prefer neutral or chaotic good.
But in Quake II I found very sick that some space marines asked to be killed after being freed and having to actually kill one to reach a secret area he was obstructing.
Edit: Ah, I also tried "Vas Kal Corp" in Ultima Underworld - The Stygian Abyss, but it destroyed everyone and everything but me, but including every item and the links between levels, so it was totally unsatisfying...
