Stalin played a very cynical and cunning game during WW2. Having deliberately killed millions of kulaks in Russia what more could the West expect when dealing with this brute of a man, a murderer of more people – his own Russians – than the Germans in total.
The full extent of Stalin’s duplicity and perfidy during WW2 was purposely understated or even whitewashed during WW2 by the British and the Americans. The reason for attaching little importance to these heinous crimes, for that is what they were, was the need to defeat the Nazis.