Well, Otoya wanted to keep that other guy, Omura, alive even though he was a monster. And Maya hasn't attacked any humans in front of him. He has no reason to believe she's an evil monster.

The important thing is: Nago got into a bath naked with Wataru. And we saw a healthy flash of thigh And the fact it's the *second* one and it seems Nago invited himself...And what the heck was it about when he was wearing the towel all girly later? XD;; And the slippers! I suspect he thinks he's training on being able to deal with being in hot water as long as Wataru can. (Which is probably hours and hours)