I mean, I guess the Genocide run in Undertale... though I still can't beat Sans. I discovered that your attacks do the most damage if you meet the requirements to spare a monster and then you attack it. I don't know if I even want to finish the game, I do mean things in games all the time but this is the only game that makes me feel bad for it.
Also, in Metal Gear Solid 3, I was doing a "Genocide Run" of that game ( not that I was basing this off of Undertale mind you, because this was long before that game came out ) and seeking out everyone I could to kill them. I ended up trying to find interesting ways to kill or mess with the enemies. I'd throw poison animals at them while holding them up, so they couldn't move, and the poison slowly took them. I'd hold them up, beat them up several times, make them think they were safe, then kill them. Shoot their limbs one at a time. Put them to sleep and then throw them into water so they'd drown in their sleep.
But the most messed up MGS3 kill was in the rocky mountain area, I'd kill a dude, let the buzzards eat him, kill those buzzards, throw the meat out so that someone else would come along and eat it, kill him, rinse and repeat until the last guy, who by association had eaten the flesh of many of his comrades. Then I just drag him off a cliff.
