We're in a day and age where people can cause tremendous harm to others using their mind alone, without throwing a punch or pulling a trigger. Just because he didn't personally cause physical harm to people doesn't mean he can't cause it to happen!
I think it's important for society to be protected from people with his mindset.
Punishing him for that without trying him for it seems like punishment without trial, which is bad. The alternative is that this punishment is just for nonviolent stuff, for which it seems excessive.