This comes off as trying to justify slavery and also being a bit lose with the facts. Not everybody in Africa was a slave, not everybody there was a canibal, and there were certainly individuals whose life was cut short by being captured as slave (particularly while in the dingy hold of a slaver ship). While some individuals might have ended up living longer this can hardly be a justification.
To Andy-it’s called fighting fire with fire.
Before the social machinery existed for slavery, captured enemy combatants were simply killed. In some cases, they were eaten.
Slavery was near universal across the world by the 1600. Much of Christendom was an exception. Christian priests with the conquistadors ruled American Indians should not be enslaved, unless they were captured in battle.
Muslims were big exporters of slavery and the idea of slavery. Many of the conquistadors' ideas about slavery came from the Muslims, who they had recently driven out of Spain.
Slavery is as old as human history. While certainly not a justification, one can’t really look unkindly upon those slave holders that were doing something that was in common practice around the world at the time.