Oh Jesus, where to start.
Maybe with the Sims, I guess. I was pretty shitty to those poor bastards. I had one guy develop hydrophobia, so I put the only food source on an island in the middle of the pool and watched him starve to death. I also routinely locked characters away to die, as I am sure we all did. I would force people to woo hoo with someone other than their spouse then force the spouse to walk in on them. I'm sure there was more, but it was long ago.
More recently I had a Skyrim play-through where I modded in all kinds of skooma-related mods and made myself the Walter White of the Nords. I mostly used kids as dealers because the guards didn't ever mess with them, but temple priests worked equally well. I spent most of my time gathering ingredients and distributing the finished product to my dealers. I also liked building up my customer base. When guards got hooked you could bribe your way out of murder for a few hits of top-notch skooma. By the end of the game I had hundreds of NPC's addicted including all of the jarls, the greybeards, and every adult in Whiterun. I gave out lots of free samples.
That got boring eventually. So I got a little darker on my next play-through. I found some mods for prostitution and figured I would make a female character that was Dragonborn by day and super-ho by night. But I couldn't really make the same money I was making peddling drugs. So I started adding in other mods that let me take some liberties with my followers. Long story short, Lydia was sworn to be my bottom bitch. Eventually I had a large stable of hos, and I made a pretty decent income.
I haven't played Skyrim in a while, maybe it's time to check the mod sites and start up a new game.
Fallout is another good option. The modding community made plenty of good stuff for those games. In Fallout 4 I was the Sith overlord of the Commonwealth. I had a lightsaber and rocked Imperial Power Armor. Another mod gave me force powers like lightning, force throw, choke, ect. No NPC was essential, and I really enjoyed force-choking Preston the first time he told me a settlement needed my help. Also had a device that let me call in orbital bombardments for situations where I just felt lazy. My companions all wore Storm Trooper armor and carried Laser and Plasma weapons modified to look like blasters. Settlers too, they were all just Storm Troopers. Basically, if you were with me you were made into a Trooper, and if you weren't with me I killed you.
The odd thing is, I am not even the worst one I know when it comes to this kind of thing. The other people I game with have done some stuff that is beyond even me. One guy found a Skyrim mod that let him harvest black souls from pregnant women. So he went around snatching the life out of unborn babies so he could have a better pair of boots, or whatever. But first the women needed to get pregnant, and that was a whole other set of mods...
Point is, that's what makes games great. They're pretend. I am not a dope peddler, or pimp, or murderer. My friend isn't running a non-consensual abortion clinic. We are really just ordinary folks living ordinary lives and having a bit of sadistic fun in the digital world.