The Question and Answer section for The Life of Olaudah Equiano is a great resource to ask questions, find answers, and discuss the novel.
I can't go through each one in this short space. The collectively teach Equiano that slavery defiles the spirit of both slave and slave owner. Slavery is contrary to logic, religion, and the human spirit.
The whites mostly used a twisted interpretation of the Bible and a form of eugenics to justify their superiority. Equino and others could easily condemn it from a common sense human viewpoint. They could also use the Bible to condemn it as well.
Some Africans engaged in human sacrifice and practiced cannibalism. They projected these fears on the white invaders who looked strange and ominous.