Yes, he can be very reasonable, even pleasant, which is one reason I put up with him for so long. Unfortunately, he turns into a vicious beast without warning, and he does it with anyone. (I am but the latest in a long, long string of "friends" he abuses in this manner.)

He's now writing Jesus fan fiction about how I'm going to go to hell for ignoring him.

So, do I seriously suspect mental illness? Yes. OTOH, some people are just uncivilized assholes. The difference is academic since even if he does have a mental illness, there's no chance he'll ever seek help for it, or entertain the possibility that anything about his behavior is problematic.