I liked it better than Elephant too, but Gus Van Sant's films are kind of strange and don't appeal to me that much (though I do like Drugstore Cowboy). The ending of The Dirties was the only part that I didn't particularly care for, but I thought the rest of the film was okay.
My only major criticism is that I had a hard time accepting that the film crew would follow Matt around as he decided to go to school and shoot people, even if they were only "the bad guys." I would expect the crew to either decide to tell an authority figure or, at the very least, refuse to get involved and follow him around as he murders people.
As for what it means, I think it's pretty straightforward: people who are bullied end up having revenge fantasies, which are violent fantasies. They do things to convince themselves that they're tougher than they are because being picked on has hurt their self-esteem. Revenge fantasies, for them, are both a way of getting payback on those who have harmed them but also at ultimately showing the world that they aren't helpless victims.