Why did Joseph Stalin execute one million returning Soviet prisoners of war at the end of WW2?

 Why did Joseph Stalin execute one million returning Soviet prisoners of war at the end of WW2?

He had issued orders prohibiting surrender (unless captured while wounded and unconscious), and declared that surrender was treason. 

So off the bat, nearly all Soviet POWs were automatically criminals and traitors under Soviet law.

 
Indeed, when Stalin’s own son was captured, he disowned him and turned down a German offer for prisoner exchange. His son eventually died in a POW camp.

All returning Soviet POWs were interrogated by the NKVD, often brutally, after which most were sent for a spell in the gulags as punishment and to rehabilitate/ reindoctrinate them into the Soviet system, and rid them of any capitalist tendencies they might have picked up abroad.

Others who had turned coat and fought alongside or otherwise collaborated with the Germans (and there were many), or were suspected of having done so, were executed.

So it wasn’t just mindless brutality - although there was plenty of that. Many Soviets actually had turned coat and fought against their country or otherwise collaborated with the enemy. That would be considered treason by any country.

 
The difference is that while other countries settled for executing the top turncoat leaders and imprisoning others before declaring an amnesty for the remainder and sweeping it under the rug, the Soviets were more bloody minded and thorough.