MA law does not allow for capital punishment. Fed law does but he should not be prosecuted that way. This in my opinion is a usurpation of the law which can be used against other individuals and groups.
What about charging him for insurrection under Fed law? I think that would be appropriate, as these Islamists have creating a caliphate as their motive and that’s the overthrow of our gov’t.
Federal jurisdiction applies as he killed and maimed people from other states and countries.
If your path prevails, then I hope that he is allowed to join the general prison population and suffers a very cruel death at the hands of his fellow inmates.