This is the classic hero complex in action.

You claim to be against X, but there is no X around, so you condition or bring someone to the breaking point to make X appealing to them, then you can claim that you were righteous all along. You create a problem and then "solve" it. In reality you did nothing, except wreck someone else's life, for the sake of your own grandiosity.

Based on what I've read, the bullies wanted the school shooting more than the supposed school shooter. Like, they genuinely wanted him to shoot them.