It was kindof an "the enemy of the enemy is my friend" sorta situation. The US may have had a war on two fronts, but so did the expanding German Empire. Hitler made the mistaken assumption that he could march right in to Moscow and take it, but the USSR did not fall to Hitler's advancing army in the latter stages of the blitzkrieg.
Post--WWII Germany was divided into East and West Germany, with the capitol city of Berlin (falling entirely within Soviet controlled East Germany) also being divided into East and West Berlin. Read up on the Berlin Airlift on wikipedia.
Anyway, Americans thought quite ill of communism in general, as it stands in what they considered dangerous contrast to our capitalistic way of life. I don't believe ANYONE would've said "let's go kill some commies" during WWII because all communistic world powers were actually our allies during the war. China was aiding us against Japan (Japan had holdings on mainland China at the time), and Russia was assisting against Germany. They may've said "kill some japs" or "kill some nazis," but I'm not sure commies were our priority. The only way I can imagine they said it and meant it was if they had misconceptions about the Nazi dictatorship.