See also the books “Tortured for Christ” and “In God’s Underground”, by Richard Wurmbrand;
and the works of Solzhenitsyn.
And even then, the Red Chinese made the Soviets look like slackers.
The proper rebuttal (IMO) of atheists who pull the “killing in the name of God” card has several prongs, depending on the initial claim.
The two big prongs are in response to an unthinking “Crusades/Inquisition” claim.
Prong #1: The Judeo-Christian God instructs us NOT to kill, particularly in His name. Note how this is not true for Muslims. Our God gives life and if mankind were 100% obedient, that’s all that He would have to do. But we’re not, and He can’t. The only thing He can do is limit the damage to life that we do, like we did in WWII with the atomic bombs.
Prong #2: If the atheist insists that death is the way of religion, and that we could do better if we gave up Judaism and Christianity, pull out the atrocities of Stalin, Mao, Pol Pot, Castro, the Kims of North Korea. If you pull out the numbers slain, and they still insist that religion is worse, then ask them how they measure “worse” and why do they do so.
If not believing in God results in tens of millions to one hundred million deaths, and believing in God results in far fewer deaths, then not believing in God is worse.