The fastest way to kill a sim

Firework in a closed room is also very fast. But take your other sim out of reach, because the fire is spreading through the walls.

Try having a dinner party with them and serve them pufferfish nigni if you have city living, make sure it's been made poorly ?

If you have seasons the fasted thing to do is put them outside in the snow without proper clothing on. or use the weather machine to make a storm and don't let them inside. If course I did the weather things to try and kill a sim I didn't like and my legacy sim decided to go outside in a thunderstorm and throw the trash away and the lighting killed her instead, so there is always a risk.

I’m a big softy and I have a hard time killing Sims even when I hate them. I usually just move them out of the house and them delete them from the manage households.

If you dont want to use mods, in my limited experience fireworks in a closed room (isolated from the main house of course) stuffed with stuff to burn seem quickest, but even that isn't foolproof. Most reliable and quickest is to use MCCC

If you want to kill an elder, send them to the gym, tire them out, make them woohoo in bed and see them die from embarrassment whilst their spouse is still sleeping and can't save them from Grimmy. At least they died happy! B)

If one of the new objects in SE is a space heater - there is a way to kill your Sims. Small room, space heater on max, lots of books and an unwanted Sim.