A soldier who fought for a terror regime is simply stripped of his citizenship. Is there anything more than needs to be said here? The enemy will kill women and children if it serves their purpose as well as butcher men by the thousand yet we simply spend thousands stripping people of their citizenship.
I can't think of a weaker response. The law must be changed to allow the country to protect itself against its citizens if they choose to undermine our system of law and order. That does not mean the avoidance of capital punishment.