Hold down LT and click on a Sim, press "Trigger Age Transition".
This will age your Sim by one stage of life (e.g Baby, Toddler, Child, Teen, Young Adult, Adult, Elder). Just keep doing this until they are an Elder, then do it one more time and they will become a ghost. Once your Sim is a ghost, you can control it or choose to "Send to Netherworld" which will make your Sim leave the game for good. You cannot retrieve your sim after this.
Another easy death - this is not a cheat but still very useful.
Make a pool and get one or more Sims to go into the pool.
While they are in the pool, get them away from the edges and go to Build/Buy mode, and go into Build and build a wall around the whole pool. This will eventually drown your Sims and turn them into ghosts.
Not a cheat but very useful. Take the Sim into a room, and take all the furniture out, including the windows and door. With this your Sim's moods will start to drop and it will starve to death.
Make a small house and cram all the furniture close together. Get your Sim to a part of the house where they can't go anywhere. Then go to Karma powers and choose "Firestorm". It will make the house catch alight and since all the furniture is close together it will all will eventually burn. Since the Sim is trapped it will most likely burn as well.
[more cheats for The Sims 3 here]