He was a very good player, and when his knee was injured, he had to quit basketball until he gets better. He was very frustrated by not playing, and started hanging out with bad guys, getting into fights, etc...

Mitsui wasn't really a bad guy; he was just trying to take revenge on Ryota Miyagi, for breaking his 2 front teeth. Thinking that getting the Shohoku basketball team disqualified would settle his and Miyagi's rivalry, he gathers a huge throng of gangsters to fight the basketball team, which included Sakuragi, Yasuda, Kogure, etc. Hope this was helpful! - Ruby -