Thanks to a bad ol' puddy-tat named Doomsday, Superman met his end. (We can be a little flippant because, like the peasant in Monty Python and the Holy Grail, Supes "got better." Also, he's a friggin' comic-book guy.)
You may or may not know that the whole killing-Superman storyline came about because DC had decided it was time to let Superman & Lois get married. However, such a big deal demanded a little cross-promotion. Why not let Superman get married in the comics, at the same time he gets married on Lois & Clark ?
Great idea, except that -- believe it or not -- comics books (at least at the turn of the 1990s) had a faster turnaround time than episodic TV.
So, to give Lois & Clark producers and writiers enough time to get the storyline in gear, it was decided to take seriously the joke that was told yearly at the annual Superman conference held by staff and editors to lay out the coming year's tales: "Let's kill him!"
And the rest, they say, is history.