Erm sorry, but I think you only heard one side of the story,
this is the other side:
South Vietnamese sources said that Lém commanded a Viet Cong
assassination platoon, which on that day had targeted South
Vietnamese National Police officers, or in their stead, the police
officers' families; these sources said that Lém was captured near
the site of a ditch holding as many as thirty-four bound and shot
bodies of police and their relatives, some of whom were the families of General Loan's
deputy and close friend. (In some accounts, the deputy was a victim
as well; in others, the number of murdered relatives were as
few as six.) Photographer Adams confirmed the South
Vietnamese account, although he was only present for the execution.
Lém's widow confirmed that her husband was a member of the Viet
Cong and she did not see him after the Tet Offensive began. Shortly
after the execution, a South Vietnamese official who had not been
present said that Lém was only a political operative.
There is some dispute as to whether the executed man was Nguyễn Vãn
Lém or Le Cong Na, a similar looking man who was also a member of
the Việt Cộng and died during the Tet Offensive. The families of
both men claimed that the Việt Cộng officer in the photo looks very
similar to their relative but neither family could say for sure
since the Việt Cộng officer's face was swollen due to severe
beating by South Vietnamese police prior to the execution. If the
executed prisoner was Le Cong Na then he was indeed only a
political operative.
Though some critics claim that Nguyễn Ngọc Loan's action violated
the Geneva Conventions for treatment of prisoners of war (Nguyễn
Vãn Lém had not been wearing a uniform nor fighting enemy soldiers
in the alleged commission of war crimes), the rights of POW status
were accorded to Việt Cộng members only if they were seized during
military operations.
In any case, the photographer himself speaks on the
picture:
The photo won Adams the 1969 Pulitzer Prize for Spot News
Photography, though he was later said to have regretted the impact
it had. The image became an anti-war icon. Concerning General
Nguyễn and his famous photograph, Eddie Adams later wrote in
Time:
The general killed the Viet Cong; I killed the general with my
camera. Still photographs are the most powerful weapon in the
world. People believe them, but photographs do lie, even without
manipulation. They are only half-truths...What the
photograph didn't say was, 'What would you do if you were the
general at that time and place on that hot day, and you caught the
so-called bad guy after he blew away one, two or three American
soldiers?'
How do you know you wouldn't have pulled the trigger
yourself?
Eddie Adams later apologized in person to General Nguyễn and his
family for the damage it did to his reputation. When General Nguyễn
died, Adams praised him as a hero of a just cause:
The guy was a hero. America should be crying. I just hate to see
him go this way, without people knowing anything about him.[2]
The bottom line is, it's easy to say this and that is an
atrocity or not, but until you are there during the heat of the
moment and are pointing a weapon and what you think is a monster
who just killed your close friend's family, you can never know what
you are going to do.
It is tempting to think that we might behave like civilized people,
but after seeing and dealing so much death one might react
differently.
The difference here was that such a moment was recorded.
Those who are anti-war don't have to look far to find a reason why
war is bad, the difference is that some are willing to pay the
price to protect the kind of soceity that makes pacifism possible
in the first place and endure the very insults from the people they
serve for it. Soldiers do terrible and crazy things in wars, but
not because most are monsters but because they are in indeed, a
terrible and crazy world.