Instructions
Take away your Sim's food source. If autonomy is enabled, click the "Eat" icon whenever it appears for your hungry Sim. Once the hunger bar decays far enough, the Sim starves to death.
Decay your Sim's health while swimming in the pool. Click "Build Mode" and delete any stairs where he could get out of the pool. Even though your pool is level on the ground, a Sim cannot climb out without the staircases. Click "Live Mode" and wait until the energy meter decays far enough, at which point the Sim falls asleep and drowns.
Start a fire in your Sim's house. Have your Sim begin making a meal, and then cancel the action once he places the meal into the oven. Because your Sim is no longer paying attention to the meal, it burns and eventually sets the oven on fire. Place a piece of furniture in front of your door or remove the smoke detector to prevent firefighters from barging in and saving your Sim from certain death.
Electrocute your Sim. Wait until a flatscreen television in your house breaks, and then select the Sim you want to kill. Choose the television and select "Repair." Provided your Sim barely has any mechanical skill, the television instantly electrocutes him to death.