Why Did Kazutora Kill Shinichiro?

For Mikey's birthday Baji and Kazutora thought to steal his dream bike for him would make him happy. So they broke into the motorcycle shop that happened to be owned by Mikey's older brother. Out of fear Kazutora hit Shinichiro in the head killing him.

Did Kazutora kill Shinichiro?

Not knowing who Shinichiro was, Kazutora rushed to save Baji by hitting Shinichiro in the head with a wrench, killing him. He was 23. Even after his demise, people continued to look up to him and celebrate him.

Why does Kazutora kill Baji?

Kazutora doesn't deal with guilt well. When he killed Shinichiro, he blamed Mikey, but now that Baji is dead, he has no one to blame but himself. He was manipulated by Hanma into killing Baji because he was tricked into believing that Baji had betrayed him, even though Baji was the only one to stay by his side.