By this point, even MC had come to realize that Susan was causing problems for everyone. He was sick of dealing with her, and when she was making Leah's life a living hell, along with breaking the law, to the point that Melody, Leah's only recent friend was willing to stick up for her, what kind of friend would MC be to stay out of it? Sure, he could have chosen not to, but he would have essentially been turning his back on his friends for a criminal, as you said, "for some ass". He didn't do it "for some ass". He was going to get that from Susan regardless. If that's all he cared about, he would have stood by her side. He was doing it to help his friend and actually make everyone's lives at school bearable. Leah and Alice were together, so anyone fucking with Leah was inviting Alice to the fight as well. Melody had also become Leah's friend at that point, too. Susan may not have caused problems for them directly, but it's clear that they are both loyal to the end. Did he have a reason beyond standing by his friend when she had a reason? No. But I probably would have done the same. Susan can be angry and want revenge, sure. But she shouldn't be in a position to get it, because she doesn't deserve it. People who seek revenge usually have a good reason. But hers is not. She brought everything onto herself. I'm not saying they should get a free pass, but at this point, the only repercussions are coming from someone who should not be in position to bring them. If she is really a cop, she shouldn't be, and she's breaking the law again, by using her position of authority. If she's not, then she's breaking the law in even more ways.
I'll leave it here, too. I'm not trying to vindicate the MC and his friends as if they are blameless, but Susan has deserved everything she's gotten.
