This movie is about choice, not morality
It's pretty simple. The bullies in this film (Who were WAY over the top) CHOSE to be evil scumbags. The outcast kids then CHOSE to torture them.
The bullies could have chosen to be human beings. The abused kids could have chosen to run away from home or whatever. It didn't happen that way. The movie is really about free will and choosing to walk down a certain path.
It doesn't really matter if bullying people is right or wrong (or harmless or traumatizing). It doesn't really matter if it's right or wrong to torture someone who psychologically abuses you. It DOES matter that everyone made the choices they did and told a story about it.