Only ~45 million people died in the 19th century due to unnatural causes, and most of those were wars, which are generally not reckoned as murders.
The Islamic slave trade is the non-war winner, at 3 million people killed.
We should note that “white men”—the British—ENDED the North Atlantic Slave Trade once and for all in the 19th century.
The Islamic world never would have, if the British hadn’t forced the issue.
Since Islam is firmly pro-slavery.