Instructions
Drown your Sim in the pool. In previous editions, simply removing the pool ladder was enough to cause a Sim to drown. In Sims 3, you must enter Build Mode and build a wall around the pool to prevent them from climbing out.
Electrocute the Sim using appliances. Directing Sims with poor mechanical skills to try to repair appliances sometimes can result in death by electrocution. This method doesn't work as frequently or reliably as the others.
Set the house on fire using the stove. Place the highly flammable rugs all around the stove, and then direct a Sim with poor cooking skills to try and cook, which can result in a fire. The fire can spread and may kill all of your Sims, so use caution.
Let time take its course. Turn the "Aging" option on, and your Sims eventually will grow old and die. This method is not as dramatic or fast, but definitely is reliable.