He's completely insane, but that's nothing new about it. If he was able to kill the parents of the guy he likes, beating up someone who wants to ruin the relationship is not something unlikely
I might be wrong but I assume he overdid it with sex because he was a virgin, because if he were to go out with every girl he would have lots of condoms at home. I don't think he's a red flag because she said several times during sex that she was enjoying it, it's not like he molested her.
The way Hwan loves Yoo-il is very disturbing and unhealthy. But Yoo-il is just too deep in this relationship that he's blind at this point.