Being a slave isn't human nature, but wanting to be a slave owner is. "Human nature" is, more often than not, a shorthand for "the parts of game theory that evolution distilled into humans"

In game theory, cooperation is a dominant strategy and evolves consistently. Slave owners and other injustice committing humans often invoke "human nature" out of trying to cope with their knowledge that it is wrong but whatever trauma or other thing makes them do it anyway, to absolve themselves of agency.

(To be clear, not targeting you personally here, but targeting the rhetoric and fallacy)