I agree, trials are necessary in all but the most blatant of cases. Someone who goes to fight for ISIS is one of those blatant cases, I think. If you leave for Syria or Iraq, and cannot explain everything you did there when you get back, then you have no business being there, and should be shot.
Ultimately, it comes down to the following: We need to pass laws forbidding Americans to even enter nations where terror groups are active without special permission from the Government. Anyone who does not HAVE that permission should be assumed to be cooperating with said terror group, and treated accordingly.