It's a well documented fact that death in the comic book world is pretty much a revolving door, with characters kicking the bucket all the time, only to pop up a few months or years later.
Half the time it will turn out Superman was really just in a coma or that it was a Doombot that died all along, but even when characters do actually die, they constantly find some magical method of resurrection. And that's not even including all the alternate universe doppelgangers who conveniently show up to take the place of their fallen counterpart.
However, once in a blue moon, dead will actually mean dead. A character will shuffle off this mortal coil and that will be that. These characters are dead and there's no magic spell, or scientific device, or even universe changing retcon that can bring them back. Some were heroes, some were villains, but all that really matters is that they've stayed dead.
Once they died, all the characters on this list were never seen again... or at least never seen again alive.