Although it wasn't canon, I have been bouncing around an idea for Apoc in my head that links his "survival of the fittest" mantra directly to the Earth X storyline. I thought it also gave Apoc even deeper meaning in the Marvel U and even more depth as a "villain".
The Celestials acc. to Earth X seeded the planet to create superhumans inorder to protect it from Galactus who would devour earth in order to destroy the celestial "egg" as it were inside.
My idea I have kicked around centers on the Celetials deal with Apoc and his acquiring their technology, incredible power and nigh cosmic intellect and understanding of genetics and biochemistry.
I figure he was given all his power on the condition that he test all of humanity, mutant and otherwise, inorder to constantly push them forward in order to ensure that when Galactus did come to feed he would be met by a race who had been pushed to be "only the strong" by Apocalypse. His mission would weed out the weak century by century making sure that only the strongest pass on their genes in the pool for the next generation. All according to the Celestials plan.
Think about it. Apoc doesn't view himself as a villain. He views himself as superior to all, working for the overall strength of all humanity (although focussed on mutants typically) and sees things amorally.
His plans fail to the eyes of the heroes, but he always manages to push them further. Even in "failure" on Attilan he managed to infect Nathan which in the end created Cable...a vastly powerful mutant and step forward in the evolution of mutant kind.
Now sure he has his own schemes. Even though he serves the Celestials out of a "debt" he despises being servant to anyone and so he has launcehed his own plans to make himself powerful enough to overcome even the celestials. Thats what "the twelve" was all about. But overall, while he does fill the role of villain (even though he is written piss poorly by most writers) he is acting in order to make human and mutant kind better and stronger.
I theorize that is his "debt" to the Celestial and they wouldn't let him die and avoid it. He has many more centuries to continue his work even though he showed he would rather die than continue to be a servant. Since in his mind being a servant, even to beings as powerful as the Celestials, means he is not the strongest and isn't fit to survive.
I could be way way off and the writers have some half cocked ******ed idea for him and his debt to the Celestials. Or I could be dead on. In which case I will never see a royalty check. LOL.