I once tried to kill a Sim... but don't you need to go to "build mode"- that is not allowed in this challenge- to do so? I once had an Evil Sim, who had the wish to see the ghost of his boss, so I went to the internet and asked how that could be done: remove the doors, they said, to keep them locked in a room. I'm still a bit curious, that one attempt I did then didn't work, the poor Sim just complained on how dirty he was but Grimm never showed up.
So, I'm assuming, locking some sims in your house, even if they don't die, wouldn't be allowed, right? That was not part of my original strategy, now I'll get more time to practice it, cause it's not working either...