Personally I don't regret this guy's death at all. As the Rev Donald Sensing put it the whole point of this war at present is to kill terrorists.
What I do regret is the way he died because it has managed to provide a propaganda coup for the enemy. Their propagandists will ignore the fact that in a reverse situation any terrorist who had found wounded US forces vulnerable would have attackedand tried to kill them the first time around and not left him to wait for 24 hours. They will now be able to claim to the world that the US is evil and uncaring. Even if this doesn't impress the ranks of terrorist recruits it will provide ammunitaion to the chattering classes who want the US out of Iraq.
The marine who killed him was undoubtedly correct tactically in his action especially given the propensity for terrorist wounded to fake it and blow themselves up along with those trying to help them. However to do it in view of an embedded journalist was strategically stupid, as a result I regret to say he'll most likely be hung out to dry as a scapegoat. As Murdoconline says:
The only thing worse than this that I can even imagine is if the Marine, instead of shooting the guy, had gone and put a pair of panties on the guy's head.