Why was Shawn such an asshole in the 90s?

Age / immaturity. He was in his late 20s / early 30s. He changed once he became a born-again Christian.

Why Shawn Michaels Was The 90's Most Hated Man In Wrestling

https://www.youtube.com/watch?v=zQU03ZSsgRA

Shawn Michaels is one of the greatest wrestling legends, with two distinct eras in WWE between the 90's and the 2000's he has entertained wrestling fans for decades. However even Shawn Michaels has admitted that during the 90's he was more than difficult to be around, with many of the WWF locker room at the time considering him to be one of the 90's most hated wrestlers.

Low self-confidence and a fragile ego. In his 2007 DVD he even talked about contemplating suicide. Add drugs to the mix and you have a recipe for disaster.

Because he threw his tag team partner thru a glass window and then went on to hire that witch Sheri as a valet and acted like an egotistical arrogant bitch. Just listen to the words of his theme song