He gets killed by Doomsday, whose a kryptonian who power is just super-strength
Superman dies as uses the last of his solar energy which powers him for one final punch to kill Doomsday. Technically he wasn't ever dead, but body went into a deep state which appeared dead, until his body could recover.
Doomsday isn't technically a kryptonian but a clone of many clones originating from a baby. Each one died from something so when re-cloned evolved immunity or defences for whatever killed the previous clone to a point it was the perfect killing machine. Doomsday also has the power to evolve after an attack. Like Superman used sonic sound and Doomsday grew protection from it over his ears.