Death by Drowning
Create a pool and have them get in. When they are in, remove the ladders or build a wall that prevents them from exiting the hottub.
Death details: They swim around getting body points until they are tired and die. A grave/urn appears at the side of the pool. If in a tub, simply box them in with walls once they're inside.

Death by Starvation
Starving a Sim is a longer process of death. You will need to remove any telephones, fridges, and pantries. If they have a phone, they will keep phoning the pizza guy for food. If they are poor, they will keep opening up the fridge and a message will pop up telling you to work for money to buy food. If you do not order more groceries then your fridge will sit empty and your sim will die. You can also starve a child for a while, but if under the age of "Teen" it will be taken away before it actually dies.
Death details: Starving Sims bitch and moan about not having food. Eventually they are unable to do anything but complain for food. They curl up and die on the floor until Death comes for them. Starving pets will whine for food, eventually curl up and die.

	Death by Fire When you are planning to kill your Sims by fire, be sure to NOT install a smoke detector or sprinkler. Put rugs by the source of the fire, as it will light up quickly and take out anyone standing near it. Sims with low cooking points using high-end appliances will cause fires more often. Things that cause fires: Fireplaces, Stoves, party fire thrower, lightning Death details: A burning Sim will scream and thrash about in the flames until Death comes for them.
	Death by Freezing. If you have the Seasons add-on, they can now freeze and fall over. They will eventually freeze to death. If you forget to put a roof on your house, the snow will get into the home and freeze them to death in there too.
	Stare at the sky and die. Have your sim go out and "Watch clouds" for a while and a satellite might just drop on its head.
	Cow Plant If you starve your cowplant, it will get hungry and try to tempt a sim with a piece of cake. If they lure in someone, they will eat them up and then your sim can go milk the cow plant.
	Reward items. Aspiration and career awards can give your sims nasty surprises if they use the items while in low aspiration levels. Some can bring your sim close to death while others will kill them completely. The exlixir of life will take days off your sims life if used while your sim has both low aspiration and mood.
	Death to Plant Sims. It seems that plant sims will only go brown and collapse instead of fully dying, which is boring. They can reproduce kids at will, and then you're stuck with a house full of plant sims that just sit in bathtubs, stand near lights and need LOVE. It would be far more interesting if they died and became compost or something, but they don't.
	Death to Kid Sims You can starve a child for a while, but if under the age of "Teen" it will be taken away before it actually dies. You can leave a baby on the ground and it will sleep there etc so you don't have to buy it a crib. You can kill a child with fire but not a toddler or a baby, even if you try to set the crib on fire with them in it. Children will get taken away if they get too cold (with Seasons expansion pack), but do not freeze to death. Toddlers left in cribs outside during the winter are also taken away after several warnings are given.
	Child services. Children left alone too long will be taken from your home. If it's a child, they will wait until after school or even the next morning to attempt to take the child. If it's a toddler or baby, they arrive within the day. It seems that they arrive within 8 hours, so do not leave your child alone at home while your sim goes to work. If the parent is abducted during the night, leaving the child alone, they will get taken away. Children taken away DO get put into adoption, and if other sims in the nieghborhood go to adopt, they just might get that child. Parents who lose a child can not adopt another.

Trapping and making miserable.
Trapping Sims in their homes is a fun way to watch them slowly die. By having no doors, they can't go get food, have friends or dispose of the trash properly. Garbage builds up, cockroaches & flies appear, and the hygiene levels drop. Bills pile up, the repo man takes things away and pretty soon your Sim is crying in an empty house.

Buying the cheapest appliances insures that they will break down quicker. Dishwashers make a huge mess when they're broken, as well as showers.

The afterlife - Some sims live on.