There comes a time in every comic book characters life when he should be killed. Usually when interest in the comic has been dead for years ,and the storylines are ancient and stale- then is the time to off him or her. This hard and fast rule only applies to comic BOOK characters. Comic STRIP characters are allowed immortality- even past their creators death. But comic book characters are a different matter.
In killing your characters- choose their end very CAREFULLY. Do NOT make their death accidental, stupid or foolish. Saving the world is great way to die. In fact its the goal of any comic book hero WORTH his mask to TRY to die saving the world. For his own self respect. A brutal death at the hands of an arch-foe is usually a good choice too. Saving innocents, or saving another hero is also a good way to die.
And in the issue where they die- be sure to have a tragic cover with their mode of death only HINTED at. A splash panel of their demise is only required at the end of that issue. BUT DYING isn't the end- OH no- its really a beginning. You need a story arc of several comics to deal with the ramifications of their death.
Its VERY important to play up the death - Try to plan an issue of your comic after the characters death with plenty of pathos and maudlin plot complications. Perhaps, where there's a funeral attended by all his friends and enemies or or where another character inspiredly assumes the hero's costume and persona. The important thing is to MILK it. You are trying to get public interest BACK onto your comic and trying to play it out while you figure out a way to bring the character back from the dead.
Because they always come back. They climb out of Reichenback Falls and spend a year in Tibet, or or reborn as another incarnation on another planet or they just had amnesia and wandered off in the first place.
Doesn't matter. Kill them - Exploit their death- and then bring'em back! That's what comics are for. That and showing busty blondes in tight spandex. It's what comics are all about.
(Sarcasm)