I would support using treason under the constitution if a case can be made. Lets not forget it has to stand up to a jury unless he pleads guilty.
I think the alleged actions are treasonous but for the victims he should be tried for murder under MA law then maybe for treason.
But the feds wont do this. They will use some statute passed by congress and probably screw it up as always.
There probably is a good Treason charge in this, which would be grounds for executing him. They would just need to tie him in some way to terrorist groups that exist in some sort of “state of war” against the U.S. Even showing sympathy with their cause might be enough when combined with his actions.