Churchill was largely responsible for starving over 4 million Indians to death, and had this to say to defend himself:

I hate Indians. They are a beastly people with a beastly religion. The famine was their own fault for breeding like rabbits.

On top of this, Churchill was very close with the Nazis, to the point that Hitler was certain Britain would side with the Nazis in the coming war. When FDR and Stalin joked about executing Nazi officers after the war was over, Churchill stormed out of the room, as "those men were defending their country."

As for the soviet prisons, an enormous number of prison deaths occured during World War II, when famine was widespread due to the Nazis storming Ukraine, the USSR's breadbasket. On the whole, soviet prisons and the justice system itself were more progressive than their peers, Mary Stevenson Callcott documented it quite well in Russian Justice.

The soviet union, despite having a progressive legal system, was in a state of constant turmoil caused by pressures both external and internal. They couldn't simply delete all previously existing ruling-class people and ideology, class struggle continues under socialism. Further, pressure from the imperialist west, invasion both in threat and in action, and intentional sabateurs meant that the prisons certainly weren't empty. The soviet union never had a single year of normal, stable growth, free from intense opposition on the outside and counter-revolutionary forces on the inside.