Jail? With as many deaths as he caused, it's pretty much a mandatory death sentence; and you know, their 90+% conviction rate all but guarantees he'll literally hang, because they still hang for executions over in Japan. The only way out of that is a determination he's mentally insufficient and could not comprehend the lethal nature of his actions... but his own statements and the premeditated nature of his crime are very indicative points toward him knowing what he was doing when he did it. Even with him showing some signs of mental illness, it's a hard sell given the context. KyoAni had affirmed they did receive a submission with his name and return address in their regular novel competitions (Free!, Violet Evergarden, and Chunibyo originate from these competitions)- so it will be hard to bank on claiming he's delusional and divorced from his own thoughts and actions in attacking them- the connection was there and this wasn't a random outburst of unhinged violence.