Making an immortal character in either DC and Marvel is essentially making a bet with every other comic creator for the rest of time. You create a character that can't be killed, and in doing so, you create an unofficial challenge for everyone else, as they see if they can find a clever loophole that lets them kill the unkillable.
This competition of sorts always serves to create a damn good comic, too, as it demands that both the writer and artist involved have to go completely outside of the box in order to create a death that feels legitimately justifiable. With these demises always being mammoth moments, there's a clear level of preparation and calculation that (generally) goes into these moments, as everyone working on them is aware that they are creating an iconic part of comic history - even if it's just one of several iconic deaths that a particular character has had.
With every immortal from Deadpool to Galactus having faced at least one actual death at some point, it's amazing to see all the elaborate and crazy ways that these 'undying' figures have been offed.
10. Kalibak - Superman: Dark Side #1/ Injustice: Year One #24
Of all the various immortal DC villains, Kalibak is perhaps the single most unfortunate of them.
This is because - though he's seldom a major villain in any storyline - he's died twice in various series and spin-offs, despite being supposedly immortal.
The first of these deaths comes in the ever under-estimated Superman: Dark Side series, where we see a version of the Man of Steel who is loyal to Darkseid, and murders Kalibak in order to appease his bloodthirsty master.
For Injustice, however, matters are significantly more simple. In issue #24, Kalibak attempts to invade Earth with his forces, seemingly unaware of the fact that this universe's Superman is one who is not going to put up with that kind of tomfoolery. So unwilling, in fact, that he slam dunks the villain into the ground with enough force to destroy his Apokalypian cranium, putting Darkseid's son down like a particularly heavy and cunning basketball.