he is NOT a God of Death, but someone WAY BEYOND a God (he can kill any god he wants with just a thought, be it a god of death, life, creation, whatever). He is the manifestation of 'The End'. That is why even those who can't die (or are immortal) are still 'killed' by him (though he is actually 'ending' them instead of 'killing'), because he doesn't need you to be 'alive' or be a 'mortal' to be able to "kill" you, and it completely depends on him what he wants to "end". So long as anyone or anything 'exists', it can be 'ended' by him at any time.