So, what do you do if you are in the middle of the war and you don't have the ability to transport the prisoner? What if holding them there may bring an attack that could get everyone killed? You're surrounded and you got the enemy wanting to come in after the VIP even though it is a fortified position, and they might be able to breach. What then? I would say at that point, "restraints" or not, the situation and the prisoner aren't exactly under control. Exedcuting them may be the only option to save the situation.
The victim was a part of a death squad. They could either execute him, which was likely their mission anyhow, and maybe catch others, or they could waste time trying to bring him back. Just because he "surrendered" doesn't mean it wasn't still a hostile war zone and he an enemy combatant.
I believe in due process, I really do. But war is hell. Sometimes you don't know what the right answer is at the moment.