This couple doesn't sound healthy at all. He just happened to be sicker than she was.
Well, we only have the killer's word that the reason why he murdered her was because she nagged too much.
I don't put too much stock in this murderer's explanation.
His wife had filed for divorce just a few days before he killed her. He probably lied to her and said he was going to change, then when she was sufficiently lulled by his lies/love-making, he took her face off with a claw hammer.