You were one of the members of the Black Dragons, the same gang that Taiju was in. Taiju seemed to be pretty intimidating, and it didn't help the fact that he was brutal and sadistic. Sometimes he would bully you, but not too much in a mean way. Though it was half a surprise (well, it was unexpected, but there's not much a difference with him being sadistic as a yandere) that Taiju was becoming a yandere for you.

Although he was hiding the reason from you because he didn't want you to be upset, he would kill more often including those close to you...

Because he loved you, and so damn much. Even if he would scare you with his attitude.