Why would a guy who already has a girlfriend like me so much? I befriended this guy who was taken obviously and besides that he became toxic, whiny jerk and insulted me and I told him to go away. It's been a few weeks and he's trying to come back..

Sounds like he just likes to play games with you. He thinks you're convenient when he wants you. You should just ignore him and find somebody free who just likes you and is interested only in you. Drop this other guy and move on, it's not worth your trouble.