The Japanese did sick stuff to the Americans, and we subsequently did sick stuff to them. Operating on American soldiers without anesthesia for training purposes in Guadalcanal was pretty horrific. So was the Bataan Death March. Americans who couldn't walk on their own were bayonetted to death.
Strangely enough, we Americans did some pretty cruel stuff to the Germans. In one battle of the Hurtgen Forest, an english speaking German officer was captured. The two enlisted men who captured him (one of which was giving the interview I was watching) did not want to walk him the 800 or so yards back to the HQ. So they told the German they were going to kill him. The German replied in english that they couldn't because he was a prisoner of war. But they shot and killed him anyway.
Found the interview, starts at 20:15
http://youtu.be/680AlBjvB2s?t=20m14s