@corpse_bride he is a loser with a sasaeng behavior because his crush didn't recognize him so he was ought for revenge... he doesn't have a personality disorder, nor psychosis. He's just a loser.
@tiniwini have you heard of depression, anxiety, trauma, etc? He's not doing it because his crush didn't recognize but because he blames Jennie taking her own life because of him. He's trying to avenge his friend in the hopes to be forgiven by her and feel less guilty of liking the same person as she did. He's obviously is need of help because he can't move on from the attachment he had to his friend and feels that perhaps by ruining the dude's life then everything will be better.