There was large difference between Eastern and Western fronts.
Generally, Western POW (British, American, French, German) were treated by their western captors according to the "laws of war", that is Geneva conventions. Of course, there were many exceptions, but as a rule they were treated decently.
This does not apply to the Soviet POW captured by the Germans and German POW captured by the Soviets. There were several reasons for this. One is enormous numbers (in the millions) of Soviet POW's in 1941. The Germans just had no means and no resources to keep this many people in decent conditions. (Some of them were simply released btw). Another, probably more important reason was the attitude of the Germans to the Soviets (and the other way around). Soviet Union did not sign the international conventions on POW. Soviet Government treated its own citizens captured by the Germans as "traitors" and did nothing to help them.
Very few Germans which were captured by the Soviets in 1941 were tortured and killed. German propaganda, on their part widely circulated this information, with photos.
This is not to say that only Soviets were guilty in these mutual cruelties. Everyone knows the infamous "commissars order" which prescribed to kill all communists, and Jews on the spot. (Some German commanders objected and did not pass this order to the troops).
(I am only addressing the POW question here, as asked. Partisans, "traitors" and general population is another matter.)