The Bullies deserved it. (spoilers)
Can we all just stop defending the bullies of the world? Bullies take sick pleasure in hurting other and taking away their dignity. The bullying presented in this film is very true to life and not at all melodramatic or exaggerated. These bullies, like the ones in real life, are not going to learn some lesson like in an after-school special. No one is ever going to punish them. Teachers don't do anything. Parents make things worse. Those bullies are gonna grow up and pass along their sadism to other people without learning a single lesson or feeling any guilt over the people they terrorized. So frankly, they deserved what they got. If you hurt and humiliate other people and take joy in it, then you deserve whatever comeuppance you receive, including being shot by your victim.
That being said, I don't support the shooting and murder of innocent people. Matt didn't hurt anyone else, just the bad guys like he said. Never hurt innocent people who've never done anything to you. Just bullies. I said it. Someone had to.