Without religion, there wouldn't be religion used as a way to stir up the masses to be utilized for fighting wars. Even Stalin loosened constraints on the Russian Orthodox Church in order to use its influence to rally Russian support.
Christianity has been responsible for the deaths of more people then all other religions, nations, races, continents, and whatever other entities you can come up with in history...... COMBINED.
But if you want to select a nation, then that would be England topping the number one spot for country as biggest kills tally of all time, and it could not have done that without the, well, blessing of the Church of England.
The second thing that Cristobal Colon did upon landing on the shore of Hispaniola was to plant a flag with a cross on it next to the flag of Spain. The day after that, the end of the population of that Island began. The Spanish rank second to the English in terms of kills in the name of Jesus. Ah, men.
Hitler was a Catholic who wanted to revive an imaginary paganism but stuck to Protestantism instead, the major religion of Germany. Jews were not the only people he had killed. Catholics too, as well as the Socialists and Communists you keep bringing up - whom he started World war II against. Add to that Slavs, Muslims, Africans - more Romani were killed in the Holocaust in proportion to their own population than any other peoples.
Most of the Founding Fathers were Agnostics at most - Deists, some may claim, euphemistically, or rather, for propaganda purposes. Granted, some were, but most were not particularly religious, nor known for any noticeable churchgoing habits. Being mostly educated men of means tended to have rendered them less likely to be groveling at the altar like the masses, whom, of course, they did not fail to notice tended to benefit from religion to inspire and help keep in control.