The following is one of the quickest methods of killing your Sim(s):
1. Create a small enclosed room away from the main house (in your Sim's backyard, for instance).
2. Insert a fireplace in this enclosure, and a door opposite the fireplace.
3. Tell your Sim to go turn on the fireplace. When he enters the room and is standing in front of the lit fire, pause the game.
4. Place wicker tables in every available tile in the room. Remove the door.
Unpause the game, and there you have it.
Oh, and by the way, if you ever try drowning your Sim, be sure to watch their bladder bar as they swim around aimlessly. Many times, I've seen it reach desperately low levels, then suddenly shoot back up to normal status...