The c'tan can't die either remember?
Didn't one get killed when they were sharding the others?
The question was who wins the lasting victory in the Materium. Immortal or not, the influence of both these classes of gods can be contained one way or another, so neither is necessarily guaranteed victory.
Well, if it's about material victory then Chaos shouldn't really be considered. A) Because the Chaos Gods themselves aren't really a player in the material realm, and have shown themselves to be largely uninterested in it. B) Because the Chaos Gods aren't in a mutually exclusive situation with everyone else: they can 'win' even if another faction wins complete control of the material realm. C) Because the Chaos Gods can't loose. Without a definitive loss state it's hard to say what a definitive win looks like.
"Look into my eyes, and see your death."
"Let them hate, so long as they fear."