Yes , the actions of both Hitler and Stalin were dispicable but wasn't Stalin on our side in the war?
The short answer is NO, he was not "on our side". He and Russia were on their own side, and we merely shared an enemy. But we were what might be called "the enemy in reserve".
The old wheeze, "The enemy of my enemy is my friend" is seriously flawed in this instance. Soviet Russia (and Stalin) was never our friend.