Enter build or buy mode. Enable the moveObjects on code. Delete the dead Sim. Or, if the Sim has disappeared (for example, when they burn to death), delete the Grim Reaper. Deleting the Grim Reaper works sometimes, but not always.
If your Sim is trapped in a fire and is too busy to do anything and you cannot get another Sim to extinguish the fire, enable the moveObjects on code and move your Sim out of the fire and onto some other area. If you want another Sim to die, simply move him or her into the fire and watch them burn to the ground.