Harry styles my bully

How is it that people can go from being best friends to enemies? That's what happened between Hanna Turner and Harry Styles.

They were best friends as kids but when they started high school everything runs down hill for Hanna.

Hanna is a beautiful girl in her last year of high school and has to deal with her bully Harry Styles. He emotionally and physically bully's her with his gang members, Louis and Zayn.

Why is Harry Styles doing this? What caused Harry to bully his old best friend? Will Harry see what damage he has caused to Hanna before its to late?