Witty Name Here said:
No need to get so patronizing, I was just pointing out how none of them seemed to be dressed for combat, one of them is wearing a Hawaiian collared shirt, and the guy holding the gun doesn't appear to be wearing any military uniform.
Atchooly, he is: an American-style fatigue shirt. This was during the second day of the Tet Offensive, involving some of the most vicious fighting of the war, and most combatants in-theater got to looking pretty rumpled by that point.
The shooter is Brigadier General Nguyen Ngoc Loan, head of South Vietnam's national police force. The deceased was, as noted above, a captain in charge of a Viet Cong murder squad specifically tasked with killing cops and their families. Since he was caught wearing civilian clothes instead of a uniform, the Geneva Conventions did not protect him from summary execution.
The American photographer long regretted taking the picture, particularly since to his knowledge Loan was a genuine Vietnamese war hero. Loan later lost his leg to machinegun fire while leading a charge against Viet Cong positions, and after being relieved of his command due to the injuries, devoted his time to setting up hospitals and the helping of Vietnamese war orphans.