It's not his fault. Really. How on earth did anyone expect him to just let that fly? Azimio purposefully pushed Rachel. Pushed her into the lockers! He saw it. With his own two eyes. Of course he was going to do something. He couldn't control the fact that doing something happened to mean punching Azimio's face in and getting suspended for fighting. Although, he's not sure it constitutes as fighting if he obliterated Azimio and came out with only bruised fists. What can he say? Rage and adrenaline can be a pretty powerful combination. Add that to the fact that he's 6' 3'' and had been working out as of late, and, well, it could be a kind of dangerous. All he knows is that Azimio, along with quite a few terrified onlookers of the beat-down, won't mess with Rachel again. Nobody is gonna mess with Finn Hudson's girl.
