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Ah, war. Yes, well we are supposed to get a declaration of war and we haven't since WWII, and killed millions. Nevermind. Back in the day Justice Brennan was the go to guy on the death penalty, because he was again it on grounds of unconstitutional cruel and unusual punishment. Marshal often backed him up with a variant dissent.
Once you join the military you give up your civilian legal status which includes most of your civil rights, and you come under the military system of justice covered by The Uniform Code of Military Justice, UCMJ. This was a fascinating monster which I wondered around at Boalt as background. I bet a lot of inactive duty national guards do not know they are still subject to the UCMJ until they are formally discharged.
Not that the provisions are followed, but there are legal guidelines about what is within rules and outside rules of war. It was pretty obvious, you were supposed to kill or disable the enemy and nobody else. The legal logic was we declare war on a nation state and fight its uniformed military. That got really furry with mass bombing raids
That got even more furry in Vietnam because the VC didn't wear uniforms. Sometime around Tet North Vietnam uniformed military units showed up below the DMZ. We never declared war against them either so it was somewhere out in neverland justified.
These particular areas of civilian and military law have been severely fucked up by Bush et al. and Obama has made it ten times worse by institutionalizing these ad hoc extra-legal practices. What is conceivably legal about a drone strike? Absolutely nothing.
During my library battles, I gained a certain respect for the military, since it was an institution of government and it was apparent back then, as an institution it was being deeply corrupted.
Ultimately, the issue isn't the death penalty at all. The issue is the corruption of a dubious republic into an imperial state under a highly centralized authority of the executive branch.
CG