You get both of the sims to be enemys...but I don't think one sim can kill the other.
But there are multiple ways to kill a sim with a cheat:
1. Type "boolprop testingcheatsenabled true" without the quotation marks
2. Shift click on the sim you want to kill
3. Click Spawn
4. Rondeys Death Creator
5. And choose one of the following! :(
Chat with our AI personalities
No, Sims can not kill each other only the player (you) can kill Sims.
No, a sim cannot murder another sim on The Sims. However, if both sims are enemies, they can hurt each other. You can also use cheats to kill your sims.
No Sims are unable to kill each other. They can however fight as much as they please and have an extreme dislike of one another.
Make more!
I think you need to be a mummy and curse them!