Yes. He had no way of knowing the atrocities that that Corporal would commit years later, and hindsight is almost always 20/20.
In this recent case with the Marine however, I believe he is more than justified given the insurgent's tactics of booby trapping their dead and wounded.
Certain rules of war, like taking prisoners, should only be followed when both sides decide to follow them. If one side decides to violate the Conventions then why should the other be forced to uphold them?