If Hitler was a perfect 10 on the evil scale, Stalin was 9.9999999999999999, but the reality is we would have had a lot tougher time doing what we did in Western Europe if the Russians hadn't done what they did against the Germans on the Eastern Front. In short, we had no alternative but to dance with that particular devil from late 1941 to the middle of 1945.
Stalin was getting crushed by Hitler. Had we not aided Stalin, he would have been destroyed along with Hitler and in all likelihood the entire Cold War could have been avoided. Patton knew this and openly advocated it, this is probably one reason that Roosevelt never allowed Patton's career to advance as far as it should have.