Interpretation of the film/ending [spoilers]
Not sure if this has been suggested before, but it's just something that occurred to me.
The "hoodies" were a manifestation of the central character's feeling of inadequacy. They're an actual physical extension of the anger and resentment that he felt towards the people in his life who didn't support him after he was the victim of a violent attack.
After his initial assault, he becomes an alcoholic, is suspended from work, and begins obsessing over youth violence and the lack of sufficient discipline in the school. Browsing through tabloid propaganda he starts to see teenagers as pure evil.
The breaking point is when he's discussing identity and perception in relation to King Lear and the kid throws the balled-up paper at the back of his head, thus reinforcing his own lack of control and his failure as a protector in the eyes of his daughter. At this point he realises how powerless he is; that's when the killing starts.
Now, I'm not saying the central character is the one doing the killings (which has been suggested by a few other posters here and elsewhere). I'm saying that his anger takes a physical form, like the monsters in David Cronenberg's film The Brood.
He conjures the evil and gives life to it; the "hoodies" are - to quote the Cronenberg film - the "shape of his rage." This is why all the victims are people who let him down, were rude to him, or were simply too apathetic to do anything about it. This is also why the central character is never harmed (even after coming face to face with one of the hoods). He is part of them and they are part of him (perception/identity). The final shot seems to be an acknowledgement of this, as he realises the rage is also directed towards his family. There is no escape from it.
To me, this was a supernatural film, though with a psychological subtext. The anger taking a physical form, as a concept, is closer to the paranormal than the psychological, though obviously I'm not suggesting these were ghosts, just a corporal representation of rage (like the dwarf could be seen as a representation of grief in the film Don't Look Now).
You're free to disagree or to offer your own theories, I'm just putting this out there as an alternative.