Sure, Germany had the Holocaust, as well as the siege of Leningrad. Sure, the U.S. had WWI and II, the Mai Lai massacre, bombardment of German cities in WWII, and the first (and second, I believe) Gulf Wars. However you could combine those numbers, and double them, and they still wouldn't meet the death toll for Josef Stalin's brutal. It is estimated that nearly 40 million people died under him alone!
Stalin killed more people than an other leader in history with his purges and man made famines. Yes the U.S. drop the atomic bomb, but its hardly inhuman as everyone died instantaneously (except any that died from radiation poisoning) and was absolutely necessary, the alternative would have been to occupy Japan which would have cost at least a million US lives and countless more Japanese lives than did the dropping of the atomic bombs.
Let's face it. The USSR did not bomb any country and killed countless of people. The two most damaging countries were Germany and the U.S. Hitler may have killed more people than the U.S., the U.S. destroyed two entire cities. I still think the atomic bombs were unnecessary and inhumane.