Revenge begets revenge. I'm sure the Bullies are going to rally behind this brutal attack regardless of his claims of righteousness in doing so. They'll come after him for it, and the people he made friends (his slice of life pals) will now become targets. We might even see some of the classmates try to join his side because they agreed with what he did (there was at least one guy that started disagree with their BS), but the more people he has on his side the, more potential people will suffer, and that will escalate to more revenge if/when they're hurt or killed. I suspect there's going to be a few casualties on both sides before it's over.
I'm not saying he's wrong. I enjoy stories of heroic justice just as much as a good revenge tale. The point of the story appears to be that he's a product of what these people did to him and every bit of suffering they attempt to visit on him will likely be reflected on them more so, unless they atone or die. I don't think he raveled in killing her, because that look at the end wasn't joy. But it wasn't regret either, more like emptiness. It's seemed like he was more about teaching legitimately unrepentant people a hard lesson and once that lesson was over both were dead and the enjoyment faded.
But, hey... loot.