kisaki actually admires hanagaki (not mikey), it mentioned in past chapter (forgot which one) where he said "farewell, my hero!" with tears on his eyes before putting bullet on hanagaki's head in the future time.
there's also the scene where kisaki as a child seeing hanagaki hopelessly fight the ones who bullies his crush (hina), i think this part is the start of everything.
a full nerd becoming full gangsta just to see hanagaki doing the hero's job.