King originally is a narcissist he hates when someone takes his spotlight it's not about Mylene as doesn't even love her. It's about him Leon took the spotlight from him by speaking those lines that is why he got mad.
Easy the MC choose to be that because he doesn't want to be on the bad side of all the classmates as they were siding with the other guy. What if he retaliated again and someone snitched on him and exaggerated the story in order to make him the bad guy?