Go ahead and execute them all.
Will that bring back any of the deceased?
No.
These things have to be prevented BEFORE they happen, and that means, among other things, sane immigration policies.
I agree.
The way to deal with this is to batten down the hatches and prevent more immigration from countries in Islamic civil wars. They can’t just escape religious civil wars, they bring them along with them.
In turn we need to get out of the way of the Islamic civil wars in their countries. There is no “our side” in an Islamic civil war.