The scale of colonial exploitation and the Atlantic slave trade trump many if not most atrocities by other groups through history.

Slavery in Africa was very different from the chattel slavery practiced in the US where slaveowners had the right to punish slaves to the point of manslaughter without reprecussion. See the Casual Killing Act of 1669.

Slaves in Africa at the time were integrated as household members or low-status laborers rather than treated as perpetual chattel. Legal frameworks allowed some slaves to marry, inherit, and sometimes ascend to military or administrative roles.

European colonists created such a demand for slaves from Africa that it destabilized local economies, promoted war through using weapons for purchase and even led locals to believe Europeans were cannibals just based on the sheer number of people they were purchasing and transporting.

I wouldn't blame Africans for being skeptical if a large group of white folk showed up at their coast and thats not even getting into the legacy of South African apartheid or the perpetuation of exploitation that persists to this day through neocolonialism.