I originally had this question after seeing the end of Jojo Rabbit where most of the surviving Nazis including children in hitler youth uniforms were rounded up to be shot (it's implied they are in the American control zone). But I've also seen this in other WW2 potrayals notably in Band of Brothers after the D-Day invasion it's strongly hinted that a number of prisoners including an ex American are executed off screen.

I've been watching the WW2 in real time series and have zero question this sort of thing was pretty regular on the Eastern Front. But honestly I always thought that simply wasn't something the Americans and British did except in really anomalous circumstances. Was it actually, if not common, at least not that unusual for the western Allies to conduct mass executions after a battle?