This famous picture by Eddie Adams was one of the causes of public opinion turning against the Vietnam War.
However, it is actually the result of a misinterpretation.
Many people mistook this as a picture of a cruel officer executing an innocent civilian.
Actually, the Viet Cong officer being shot was reported to have knifed to death the family of a military officer in South Vietnam after the 1968 Tet Offensive. So he deserved to be executed.
The executing officer, Nguyen Ngoc Loan, later went to the US when the South fell. He died in 1998, exactly 30 years later. Photographer Eddie Adams later made a public apology, and said Nguyen should have been hailed as a hero.
Remember, don't always trust the pictures at face value. The real action happens outside of the frames.