Everyone's complaining that both semes in this story are horrible. Not everyone has a happy story. People date abusive person after abusive person in real life to. Of course we want to see him escape and be free, and yes the seme is deranged. It's painful to read, but we can't have a knight in shining armor and happy ever afters everytime. Stories that are realistic can be pretty gruesome and bitter.
Does he cheat a second time later in the story?
So it seems. Two times with the fiancé, but I'm not sure, maybe the novel it's more clear.