(1) Make them swim in a pool and remove the ladders. This was very effective in the Sims 2, however Sims in the third instalment have advanced to climb out without ladders, so it is now necessary to build a wall around the pool (after the sim is in it) to drown them effectively.
(2) Death by Electrocution: make a sim with no mechanical skill fix an object. They will become singed the first time. Without letting them recover or shower, make them do it again. They will blow up and suffer a hot and violent death.
(3) Eat magic beans. You need the Sims 3 Supernatural Expansion pack to unlock this item. You can never tell what the beans will do to you when you eat them, but there is a small chance of death.
(4) Fire, fire! Lock Sims in a room with a bad cook, a cheap stove and lots of cramped objects without a door (no fire alarm!) and watch them burn, baby!
(5) Death by Incapacitation: lock them in a room with no toilet, no fridge, no bed and watch them starve to death.