Take a look at Koschei the Immortal. I don't know if that's the flavor your're looking for, but it's Russian, it's mythical, and it's not very nice.
Fluff-wise, I think this is your biggest red flag becuase it's unique. It's a neat idea, but the existing chapters all either view the Emperor as a divine warrior or as an exemplary man - none view him in this light. Could there be an event in the chapter's history or a specific element of Aggelos' native culture that would lead to this cult?
Why not? The greater the internal conflict, the farther it can carry the story. A bunch of puritan Blood Angel successors that are even more susceptible to the Black Rage and other defects strikes me as interesting.
*Edited for grammar.