When it comes to "personal" incidents I think it's more of a matter of "payback", although "payback" could be done on far larger country vs country payback but I do not believe that was the case of the US dropping atomic bombs on Japan...that was a different story.
There were many incidents of soldiers killing snipers, flamethrower operators, minelayer sappers, etc. and their superiors simply looked the other way.
One thing you have to do with history is get every fact possible and then ask yourself (after long soul searching) of WHAT WOULD YOU HAVE DONE?
Now that's one tough pill to swallow, much less really want to come face to face with. I can only imagine training with a company of guys for months, being shipped out, sent into real battle, and seeing them picked off or mowed down by a specific enemy soldier and not wanting payback--even if the guy did run out of ammo and food. I can't say I'd judge that guy too harshly, and as said before, many superiors looked the other way and the incident went unreported. Unethical or even criminal? Of course. Something that has happened in most wars since the dawn of time? Yes.
Country vs country "payback" has many ugly faces but the US was on the world stage as a superpower for really the first time nearing the end of WWII and the eyes of the world was on them and Truman knew it. The best estimates Truman was getting from Bedell Smith was that "Operation Olympic" would cost at least 1 MILLION American lives to invade the Japanese homeland and bring a conventional end to the war with Japan. On the other hand he was then (after Roosevelt's death) finally and fully aware of the US A-bomb project and it's possible ability to save over 1 Million American lives at a time where already far too many US families had made sacrifices. At that point, I don't think payback for Pearl Harbor was on his mind at all...finally ending that horrible conflict as quickly as possible was...along with keeping as much of Japan out of Stalin's hands, who had only declared war on Japan days before.
Yes, there is an "other hand" to this as well. Stalin was so incensed about Soviet losses in "Operation Barbarossa" and the atrocities, not to mention the USSR having taken the hugest losses of soldiers and civilians in the war, wanted payback on Germany on a colossal scale. Of the over 1/2 million German POW's taken (some sources estimate ever far more) less than 100,000 (and some sources estimate far less as well) were ever repatriated back to Germany after the war was over. Their fates are mostly undocumented but gulags, death camps, and out and out execution has been linked by ex-camp guards after the fall of the USSR. Payback in Stalin's eyes also had much to do with Hitler's "shoot all Soviet political commisars" on sight order as well as Soviet POW's being sent to German extermination camps...and payback he got.
With over 60 years of 20/20 hindsight, it's difficult if not impossible to put yourself in the shoes of a soldier who sees death all around him everyday and second guess and judge them without actually have been in THEIR shoes at the time.
