<Snipped quote by t2wave>
He was 17 at the time and influenced by the mutterings of an Alien Warlord from another planet.
But tried as an adult means many of the protections that a minor would receive would be waived. He would also have to explain that an alien made him do it. At which point it would be more likely that he would be committed or snagged by the government for study.
Though I suppose it's not really critical.