Bangchn is in the mental hospital because of how many murders and kidnaps he did. He hates you because your the one who turned him in to the mental hospital. And also because you were talking to other guys and being around other guys

Bangchan was sitting on the floor with headphones on listening to music in his room (mental hospital). He was listening to rebel music and didn't care about anything else around him. The doctors and nurses all scared of him