In re reading WoK and WoR in preparation for OB I started thinking that humans were the voidbringers to start with. When the reveal came I was not surprised at all. The listener songs talk about the spren betrayal, choosing humans, how they didn't shatter the plains, and a few other hints that could be picked up on.
I don't think, however, that odium really was their god when they showed up. I think he followed them after killing their god, or perhaps both of their gods.