We would never use the death penalty with them. Put them in jail and then release them two years later and say they have changed. They go back to terror again and hurt Americans.
The democratic way
I kind of like that approach:
Bring them to the US, put them in jail for a few weeks but give them special treatment: segregated, comfortable accommodations and excellent food— eventually moving them into a halfway house. Then hold a mock trial and plant the story —un-named source to Al Jazera— that they pleaded to a reduced charge and were sentenced to time-served.
Send out a few missiles and kill a few Jihadi associates.
Then send them back to Syria.