Damian,
The Communists also had their own dogmas, heresy, martyrs, inquisitions, iconography, and rituals, even a form of Pseudo-science.
These guys killed people who didn't cooperate with their regimes. The Russians probably attacked the churches and clergy because of their cooperation with the Czar, but still used the church for support in WW II when it became The Great Patriotic War.
Like the Nazis, the Communists came along in a time of increased efficiency in modes of destruction.
Going only as far back as 1914-18 one sees that a collection of divine right monarchies and religiously strong democracies could wade chest deep into war with the beginnings of planes, machine guns, tanks, gas, high explosives, and submarines. 16 Million deaths and 20 million wounded (wikipedia) by folks that did claim to believe in various higher powers and morals. One can see from the iconography in these beautiful cemeteries of this war that there is hope for an afterlife and that the dead died for God and Country.
Go to WW II and all of the powers except Soviet Russian, which still had the Orthodox Church. "Good" or "Bad" most of the destruction was done by theists.
I can't say how many more would have been killed in La Conquista, the Crusades, or the 30 Years War if the troops had machine guns or gas but we can see that the majority of the forces who did possess them in the 20th Century were theist.