One time I entrapped a single mother and her son in the kitchen and removed all appliances. I watched them starve. I also locked up 4 innocent sims and only let them out for group showers. Food was delivered to their cell as they watched the warden live the life in his luxury office and sometimes he performed tests on them.

I don't care enough about the Sims 4 Sims to be cruel to them. It's like a hunter hunting taxidermy instead of live deer.

I kept clones locked in the basement lab of my Selfsim's son who is a scientist. He would use serums and the ray gun on them and would laugh about it.... despite being a good sim. I purposely fed a sim to a cowplant. Not much really, I'm fairly tame.

Over a year ago, I did a silly little add-on mod for a bigger science mod, a new machine that can help spawn the collectable animals. I also made it randomly spawn the otherwise useless matter cube that came with GTW, the one that spawns by default when using the cloning machine. I gave the matter cube a new function, so sims could pick it up and eat it for a new buff and even more collectables spawning in their inventories for their bravery in eating the mysterious matter cube. Any road up, that little pre-ramble needed to explain the despicably disgusting thing I made sims do using the same method. Enter Cats & Dogs. Enter Doggy Poo. Giving it very similar code I made for the matter cube. Did they pick up the dog shit and eat it? Of course they did. So disgusting I could not bring myself to fine tune it or ever consider releasing it. And the poor sim I forced to undergo this mod test, I can never look them straight in the eye again. Despicable me.

It was that last line that really did it for me!

During the TS3 era, my favorite thing was to summon meteor showers and take out as many sims as possible, then go and collect the space rocks and scrap for profit after.

The closest thing I have found to that in 4 is vampires with maxed out emotional influence. My record as to date (it was a couple months back) was 30 sims in one night at a dance club. He kept firing off angry auras and some of them would drop immediately. He'd meditate for a bit, recharge, and fire off some more. He reduced over half the town's population because I sent him out several nights and sometimes it would be 9 or 10 deaths, other nights it would be more.

I think for this game the worst I've done was throw a party
Start a fire with a cheat then trap everyone inside.
I was really bored one day

In a moment of cruelty and immaturely I made two elders woohoo till on them died. I then wrote "banged to death" on the tombstone.

Drowned a few unwanted sims and made friends with the Grim Reaper all in one visit. Being friends with Grim is sociopathic in its own right.

I think the most interesting death I had was during the Sims 2 era. Curious brothers had a double wedding. Vidcund made a wedding cake and on their way to go to the honeymoon in the limo, the two couples were in the limo, the house caught on fire, then somehow the fire jumped to the limo that was so far away from the house and everyone died except Pascal's alien daughter Nebula.