There are many rumored ways to die in the game which I have yet to find proof of.

Heat - OK I was wrong about this one. Contrary to my previous statement, there is a very small chance an overheated sim can spontaneously combust. However though the fire might kill the sim, heatstroke itself is not directly fatal.

Cold - While passed out from cold a sim can die of hunger but cannot die directly from being frozen.

Evil Kites - Kites flown in a lightning storm have a 30%, 50% or 70% chance of attracting lightning. This is true for any kite, not just evil ones, and the kite is not directly responsible for death.

Elevators - Falling in an elevator causes -30 comfort, -20 bladder, -10 energy, -30 fun, -10 hygiene (the latter not for plantsims). There is no death check and none of these motive drops can be fatal in and of themselves.

Nothing - Sims don't die just for the heck of it. Even the "death by pregnancy" bug in the base game was triggered by sims being sick and was a disease death. If your sim died, there was a reason.