I don't think that Moon is initially initially evil, because he is cute enough on the advertisement with sleepy sweets, most likely this is unreliable :>
when the book comes out, then we'll find out, just wait until December 6
This info I’m getting is from one of the books that’s being discussed on the Freddit discord (some people got the books earlier). There might be more context behind it, or additional info in the other books, and I’m happy to go digging to see if anything has been revealed for Somniphobia etc as of yet if anyone’s interested as well. But the info those of us that don’t have the books have is still limited without an actual physical copy of the book in front of us and I’m sure more info will probably come out in time too like you’ve said!
That being said, I also agree that Moon being actually evil hasn’t been confirmed by this passage from one of the books because:
1) If Moon was truly evil prior to the virus the theatre would have held approximately 1 show before getting abruptly shut down (seriously, how would that theatre survive if Moon’s just going around routinely hurting people as a known gimmick? That would be a liability even for Freddy’s)
2) Like you said, why would they have merch of Moon if he was evil and horrible by default? Just because he plays a role in relation to Sun at the theatre doesn’t mean he’s actually a bad person. It would make more sense if he’s just playing a role to entertain the kids.
3) The book lore really doesn’t have the best track record of matching up with the lore of the games at the best of times so I wouldn’t take anything the books say to be officially canon as far as the game verse is concerned anyways.