Sorry, Heng, but IMHO you called it wrong here.
The area had been cleared, but it had then been reoccupied by
insurgents - it would not have been sound to assume that the area
was safe and that the wounded were still unarmed.
Also, the definition of "armed" isn't restricted to firearms, is
it? The wounded soldier could have decided to have his bit of glory
and be a suicide bomber - not an unrealistic scenario in those
parts. From the image posted in this thread, there's no way of
telling if the wounded soldier was covering an explosive
device.
Given the circumstances, especially vis-a-vis the "false white
flag" technique, the situation does provide mitigating
circumstances. While the My Lai comparison is inappropriate, this
situation may be compared to the German use of the "false white
flag" technique in WW2 - after a while, the Allies learned to
disregard the white flag and were it was never held against them -
the common sense of their approach prevailed.
If the other guys want to use the white flag for deception, they've
got no business bleating about it (or anyone on their behalf) when
it turns around and bites them in the ass.