It's always easy to trot out the generalities about "everyone does it", which is why people always do. The history is there for those who want to study it and really understand what happened and why.
After the massacres of Canadian POW's by the SS, a lot of those goons got shot whether trying to surrender or not. No loss in my book. They were massacring British prisoners in 1940 and kept doing it right through the war. Not to mention all the Poles, Belgians, French, Czechs, Dutch, Russians, Greeks, Italians. You get the picture?
My Lai was an eternal disgrace to the USA, and in WWII there were thousands of My Lais. I'm not aware of any committed by the Western Allies however. And to those who got their history from "The Valor and the Horror",

save your breath!
It's interesting to see the generational breakdown here. Those who were old enough to be there have no illusions, while most of those of my age, don't know or don't care, or both. Sad. Those vets should have talked more.