An exceptional amount of psychopathic fun can be had with the chickens and cows in Resident Evil 4 your first time through the village and its farm, if you have a couple of grenades handy. The infinite ammorocket launcher packs a lot more punch, but you'll likely be causing wanton destruction to everything in sight at that point.
Damage the cows enough and they'll damn near kill you.
Don't forget the good ol' Merchant, who can get killed by firepower, but'll re-spawn in the next area. I don't have enough cash for that weapon upgrade, eh? Enjoy a face full of rocket then!HAHAHAHA!!!
Unless you're on Professional, in which case if you kill him he never respawns. Whoops.
Obs Cure: The Aftermath will let you bludgeon your partner as much as you want, with baseball bats, hockey sticks, and more. All you get for it is a spray of blood and an indignant response, though, it doesn't actually hurt them... but that's a good thing when the two of you are fighting the undead in close quarters. (And oddly enough, projectile weapons don't have any effect on your partner.)
Haunting Ground: The alchemy room allows Fiona to forge items for herself and Hewie. Most are helpful (reduce Panic/raise Stamina) - but some do the opposite (one of Hewie's items actually causes him to become aggressive towards Fiona - even at the highest possible relationship level). Using such items or attacking Hewie directly is actually necessary to get the bad ending (and no, just ignoring him isn't sufficient; his relationship value has to be low enough for him to attack Fiona to get the ending).
Also, there are various death traps throughout the game; most are glaringly obvious and there just to see Fiona die in a creative way/unlock cutscenes in the Bonus Room.
Hell Night doesn't seem like a game that could have the potential, but since your partners die as soon as you're caught by the monster, that's been chasing the player constantly, it's possible just to get hit on purpose. If you can spend time enough and even survive long enough to return to the Mesh (a safe hub-level), you can kill off all the partners in the game. You get a bad ending, too.
Plenty of this in the STALKER series. Oh look, a wounded guy on the ground. He needs a medkit. Do you heal him and give up valuable equipment, or finish him off (or let the local wildlife kill him) to loot his corpse? Or heal him, trade with him, then shoot him and loot him? Or even better, heal him, trade with him, then let him be torn to shreds by that pack of wild dogs over there and then loot his corpse? Hey, there's a guy who just helped you out. Do you thank him and leave him be, or shoot him in the face now that he's of no further use so you can get the unique weapon he's carrying? Do you warn those Loners that a group of Bandits are going to try to kill them, or side with the bandits themselves?
If you play your cards right in the "End of the Road" scenario in Resident Evil: Outbreak File #2, you'll be followed around by Linda, an NPC scientist who moves slowly due to a gunshot wound to the leg. If she's struck by enemy attack, she's simply knocked over and does not appear to take damage. This makes it possible to run ahead of her and use her to clear minefields for you.