I can understand why the count sacrificed his wife. She literally betrayed her husband. While he is out there killing heretics and keeping his family safe at home. But comes back home to see his wife doing heretics stuff.
He kills everyone except her.
Her smile stops him from killing her. He accepted defeat but couldn't.
He almost kills himself, but then he sees the weird egg thing. Which called those god hands.
He was so weak and couldn't bear to live with a wife who did satanic things.
So he turned into a monster.