A picture often does not tell the complete or even accurate story.
That is a fact.
That image was used by the anti Vietnam war movement to show how horrible this war was.
But if you tell the reader that the guy about to get shot in the head was a Viet Cong captain that had been caught murdering the family of members of the national police in Saigon during the Tet Offensive of 1968, suddenly your sympathy for this guy fades.
Context matters.a
Saigon, February 1, 1968, Associated Press
photojournalist Eddie Adams won a Pulitzer Prize for this:
Red6: "But if you tell the reader that the guy about to get shot in the head was a Viet Cong captain that had been caught murdering the family of members of the national police in Saigon during the Tet Offensive of 1968, suddenly your sympathy for this guy fades.
Context matters."
Worth repeating -- this image helped defeat the American military in Vietnam.
It's because the sad fact was: by the end of 1968, American leftist news media (i.e., Walter Cronkite) had convinced the public that winning in Vietnam was not only not possible, but also not even desirable, and this photo helped make that case.
Here's another that clinched the argument in 1972:
Tanken by Nick Ut on June 8, 1972 -- Kim Phúc, a nine-year-old girl, running down a road after being severely burned by napalm dropped by South Vietnamese planes.
This photo won Nick Ut a Pulitzer Prize in 1973.
It helped clinch the argument in favor of American defeat in Vietnam.
Napalm girl, 1972 and today living in Ontario, Canada:

Kim Phúc is a UNESCO Goodwill Ambassador and runs a non-profit organization to help children affected by war.