Although the attackers who cause mass deaths are often thought to be immigrants, sometimes locals can also commit this act. What could be the killer's aim in attacks that are not carried out on behalf of an organization? Most likely revenge on society. I can't find any other reason. The attacker wants to destroy the society that hurt, excluded, humiliated and polarized him.
I have no idea who caused this event. It doesn't matter though - it is still pathetic. They want some celebrity, they want to feel relevant, even if it kills people and themselves to be so. It doesn't matter if for an organisation or the self, it is still an act of wanting to belong to something.