You called the necrotheists atheists in the op just now. I think that may not be the appropriate word. Especially given the name you've given the people themselves.
Atheists don't believe there are gods. Is this the case? If not, Antitheist would be more appropriate, since it would roughly mean "against the gods." Necrotheist would roughly mean "Death is god." Though the latin kinda breaks down well before this point since necromancy roughly means "talking to the dead."
Sorry for being overly pendantic.