If atheism caused murder, then Sweden and Japan would be the murder capitals of the world. Actually, it's Christian countries like Guatemala and Brazil that have the highest rates.
No, what murdered tens of millions in the Soviet Union, China, and Cambodia was a fanatical devotion to Marxism, and not the Groucho variety. Marxism isn't just a few ideas about government; it's a quasi-religion that pretends to explain Man and his role in the world. Marxist fanatics and religious fanatics might think they're on opposite sides of the spiritual spectrum, but it's basically the same personality disorder.
Still don't understand? If you ever lived in Stalingrad or Stalinabad, walked along the Stalin River, climbed in the Stalin Mountains, or lived in any one of a zillion villages with a statue of Stalin, you might get the silly idea that Stalin is just another messianic figure. And you'd be right. The same brainless sheep who mindlessly follow the Russian Orthodox Church are the same brainless sheep who mindlessly follow Uncle Joe.