List of Sims 3 Deaths Possible

Bold is the name of the death, by the way.

Base Game to Pets

Old Age Most life states have limited lifespans, and will die when reaching the Elder life stage. SimBots (Ambitions), Vampires (Late Night/Supernatural) with the Immortal lifetime reward (Supernatural), Sims with the Age Freeze Potion lifetime reward (Generations?) and Plumbots (Into the Future) cannot die this way. This is also the only way pets (Pets) can die. This death can be countered by disabling the Sims Age setting in Gameplay Settings.
Starvation Most life states will die if their hunger need reaches critical amounts. This death cannot be countered without cheats. Vampires (Late Night/Supernatural) have a similar death in place. SimBots, Plumbots, and PlantSims cannot die this way. Children and younger, alongside pets (Pets) will instead be taken away by the social worker.
Fire If a Sim has the On Fire! moodlet active for 1 hour (3 for Daredevils and Pyromaniacs), they will die. Sims at levels 7 and above in the Firefighter profession (Ambitions) and Genies (Showtime) are immune to fire, and cannot die this way.
Electrocution There is a chance for Sims to die when electrocuted.
Drowning If a Sim has the Drowning moodlet, they will die if it expires. Oddly, Sims with the Lungs of Steel lifetime reward (Island Paradise) can still die this way.
Mummy's Curse (World Adventures) If a Sim lingers with the Mummy's Curse for two straight weeks, they will die by turning to dust if cured.
Meteor (Ambitions) If a Sim is standing outside, there is an extremely small chance in which a note says it has suddenly gotten dark. If the Sim doesn't move on time, a meteor will strike at the position they were, killing them. Sims close to a deadly position can get singed or burned. This is one of the most dangerous deaths that can occur without cheats.
Thirst (Late Night/Supernatural) If a vampire Sim's thirst reaches critical levels, they will die by turning to ash. This replaces the Starvation death for them.

Showtime to Into the Future

Human Statue (Showtime) Magicians have a chance of dying if they fail the Buried Alive trick from the Box of Danger, unlocked at level 7 in the said profession. The Unlucky trait cannot counter this death.
Watery Grave (Showtime) Magicians also have a chance from dying if they fail the Watery Grave trick unlocked at level 9 in the said profession, also done in the Box of Danger. Unlike the previous death, the Unlucky trait CAN counter this death.
Jelly Bean (Supernatural) One effect of a jelly bean from the Magic Bean Bush has a 1% chance of killing the Sim who eats it.
Transmutation (Supernatural) Use of the Philosopher's Stone has a chance to kill the user by turning them into a golden statue. The statue is also worth §50,000 if sold. This is the only death that requires a lifetime reward to occur in the game so far.
Haunting Curse (Supernatural) If a Witch's Haunting Curse spell, unlocked at level 5 in the spellcasting hidden skill, backfires, there's a chance for the caster to die of hunger. This can be countered by the Magic Hands lifetime reward, which also makes spells never backfire.
Freezing (Seasons) Sims who stay out in the cold for too long can die if prolonged or starved. This can be countered by the Immune to Cold lifetime reward.
Blunt Force Trauma (University Life) Sims who fail to open a murphy bed or slam/shake a vending machine for free snacks or drinks may be crushed to death.
Ranting About Death (University Life) If a Sim rants about death on a megaphone, the Grim Reaper will give them a stern warning, and the Sim will be given a moodlet for 12 hours. If they rant about death again before the moodlet expires, the Grim Reaper will take away their life.
Running Out of Oxygen (Island Paradise) While scuba diving, Sims can run out of oxygen if they stay too long or starve. The Lungs of Steel lifetime reward can make Sims immune to this death. Mermaids cannot die this way.
Shark Attack (Island Paradise) Also while scuba diving, sharks are a hazard. There is a chance for the Sim to be killed, leaving some of their bones, including their ribcage behind. Mermaids are immune to this death, as they are ignored by sharks.
Dehydration (Island Paradise) If a Mermaid Sim's hydration need gets low, they will pass out. If others don't splash on the mermaid to refill their hydration, they will die.
Time Anomaly (Into the Future) When a Sim without the Time Immunity lifetime reward time travels, there's a chance for them to catch time paradox sickness. They will fade out of the time continuum if not treated at the hospital.
Jetpack Crash/Falling from the Sky (Into the Future) There is a 25% chance for Sims with a low Advanced Technology skill to crash when flying around with a jetpack, with a chance to kill them.