In this context, I don't see why the answer would need to be any more detailed or nuanced than the first paragraph of the Wikipedia page.
Criminal prosecution is governed by law, and arbitrarily executing citizens (specifically, per Senator Paul's language over the course of the filibuster, those who are not actively engaged in combat) is a pretty direct abrogation of rights granted by said law.
You can be involved in military operations without being involved in actual combat. Even during WWII that described about 95% of the life of an infantryman (to say nothing of those who never went to the front lines).
And either way once you've put yourself in a military status in conflict with another power you've lost the right to have to agree about where the next battle starts; either party gets to decide until a later peace agreement/cease-fire/etc. is reached.
Criminal prosecution is governed by law, and arbitrarily executing citizens (specifically, per Senator Paul's language over the course of the filibuster, those who are not actively engaged in combat) is a pretty direct abrogation of rights granted by said law.
What's the real question here?