KOTOR has quite alot of cruelty potential. I did some very nasty things to get the Sith ending. Like wiping out entire villages, machievellian subterfuge, torture, etc.
Fallout New Vegas has a wide breadth of evil acts you can do. Randomly killing named NPCs, stealing, cannibalism. I played a psychopathic character and carried out almost every act of betrayal and cruelty imaginable. You can side with a particularly evil faction. Many manic evil laughs were had. BWAHAHAHAHAHAHAHA! However, the game doesn't stroke your villainous ego much. The karma meter is flawed in that it's relatively easy to get back to neutral and good karma just by doing a few quests, even if you just mercilessly slaughtered several towns.