You have nothing to support the idea that “atheism” has killed more people than religion. It’s a ridiculous remark not the least of reasons because “atheism” isn’t a belief system. North Korea isn’t “atheist”; Kim Jong Un is their god. Same with the worship of the state required in the USSR and Cambodia. Those weren’t “atheist” states. They were totalitarian/authoritarian dictatorships where the dictator replaces god. It’s replacing one god for another.
As for your canard about religion ending slavery, Frederick Douglass disagrees:
https://www.newyorker.com/books/under-review/american-christianitys-white-supremacy-problem
Early on in “Narrative of the Life of Frederick Douglass,” the first of three autobiographies Douglass wrote over his lifetime, he recounts what happened—or, perhaps more accurately, what didn’t happen—after his master, Thomas Auld, became a Christian believer at a Methodist camp meeting. Douglass had harbored the hope that Auld’s conversion, in August, 1832, might lead him to emancipate his slaves, or at least “make him more kind and humane.” Instead, Douglass writes, “If it had any effect on his character, it made him more cruel and hateful in all his ways.” Auld was ostentatious about his piety—praying “morning, noon, and night,” participating in revivals, and opening his home to travelling preachers—but he used his faith as license to inflict pain and suffering upon his slaves. “I have seen him tie up a lame young woman, and whip her with a heavy cowskin upon her naked shoulders, causing the warm red blood to drip; and, in justification of the bloody deed, he would quote this passage of Scripture—‘He that knoweth his master’s will, and doeth it not, shall be beaten with many stripes,’ ” Douglass writes.
Lest you think this is a vestige or days gone by, you should be aware that modern Christians are no better than Auld:
In a 2019 nationwide survey, eighty-six per cent of white evangelical Protestants and seventy per cent of both white mainline Protestants and white Catholics said that the “Confederate flag is more a symbol of Southern pride than of racism”; nearly two-thirds of white Christians over all said that killings of African-American men by the police are isolated incidents rather than part of a broader pattern of mistreatment; and more than six in ten white Christians disagreed with the statement that “generations of slavery and discrimination have created conditions that make it difficult for blacks to work their way out of the lower class.”