Yeah he hacked minecraft services to generate a profit when he was 19 or 20, I wish I could do that when I was 19. I dont think I would have known better at the time either.
He's as responsible of taking down Dyn as Mikhail Kalashnikov is responsible for all the people killed by the ak-47. He simply open sourced a tool for other people to use however they like.
He took the plea to reduce the sentence and the legal expense.