In WW2 it was frowned upon to shoot an enemy chute, because in real-life you'd be taking another man's life when he was helpless, or not able to defend himself.
It was publically frowned upon but done by all sides in the war. Nor do I believe there is any record of any pilot on any side that was court martialed or faced any other type of disciplinary action for doing so. When the bullets start flying and people start dying, any "gentleman rules" to combat are quicky chucked out of the window.
Hell, one of our presidents willingly strafed life boats full of Japanese sailors in the Pacific.