Or the soviets did to believers, for that matter

while tribal hunters simply killed for access to the best hunting grounds? Mongols killed why? IMO you can reconstruct this line of thought easily -- humans killed other humans brutally and without fail; some humans interpreted the world in divine terms and guarded fertility; Religion combines many strands with intention, while the killing for other reasons does not cease.

Don't forget the crusades.

The first crusade in 1096–1099 was to drive out the muslim colonizers from the holy land