I think I need to clarify.

C'tan are not "gods", they are physical entities of immense power, encased in living metal. They are referred to as "gods" only because of their incredible powers (in the same way the Primarchs are often referred to as "gods"in comparison to normal men), not because they are actually divine. The night bringer isn't the concept or embodiment of death, he was originally detailed as influencing many species "perception" of death as an entity.

In essence he is just an incredibly powerful being that appeared on many worlds to many species over the course thousands of years, causing the primitive species to attribute its ruthless attitude as being a manifestation of "death".