Aikawa doesn't look like he knows about Risu's death so it must've been somebody else who killed him. Possibly Ai? If we assume that he's the host, he might have access to other alters ' memories.
If the boss would kill a comrade for curse magic, why didn't he do the same to Nikaido; either Kaiman is still a predominant personality or the other personalities don't have the access to the exact same memories.