@Michael Watkins
The thing is Norman's behavior does not meet the legal threshold of clinical insanity, not once his personality merges with that of the Green Goblin. He knows what he is doing is wrong, or at least that other people would consider his actions wrong, he's even said so on different occasions. He simply does not care. There is thus no way a court would find Norman not guilty of his crimes thus he is responsible for his actions (post-personality merge).