Why did CM punk turn bad????

The fans chose Jeff Hardy over CM Punk so he decided to turn heel and insult anybody who supported Jeff Hardy.

he's not really bad it's just a storyline between him and jeff hardy

It was because of the fans and Jeff Hardy .
Punk felt that he was a villian when hee cashed his briefcase at Extreme rules so he might as well be a villian if the fans are going to prefer Jeff over him

hes mad