Kamui was never strong enough to defeat Fuuma. He could never have bared to hurt his friend. And that was why Kamui's sudden death did not surprise Subaru at all. He'd been found one morning in his bed, decapitated and with a smile on his face. A grief stricken Fuuma - not Dark Kamui - came by soon after. Subaru followed, mildly interested in what had returned Fuuma to himself. When he saw the body, the current Sakurazukamori just lit a cigarette.
"We should have realized that he'd be a true Megami."
He was buried later that day, next to Kotori and Nataku, his wish fulfilled.