Not self defense, as I doubt the guy was still standing and an active threat somewhere halfway. Neither do I subscribe to the silly American logic that you can just kill anyone who gets in your way. Besides, he brought along a knife with the explicit and only motive of committing murder upon that bully. It was pre-meditated.
But at the same time, some bullies do so much to their victims that psychologically, there is no way out except violence. Think of cases where a very abusive husband is killed by his wife because it was her only way out.
Judging by the amount of violence, gang-form and public accouncement the bullies made, this is a case where a gang of bullies goes very very far.
This is a typical case where you need a verdict of not guilty to manslaughter by reason of psychological trauma, but a sort of conviction for using a weapon.
Damn, US legal systems are so strange. If this was the Netherlands he'd been received a zero punishment verdict for manslaughter, and a conviction for illegal possession of a weapon. That would likely be a few dozen hours of community service, or a suspended prison sentence.