Throughout other works about the relationship between white men and aboriginal peoples, the white man is described as civil, while the aboriginal man is described as savage. Interestingly, in Newton's work, he uses the phrase "white savage," implying that those who call the native man a savage are mistaken (Newton 20).
Newton wrote this account to help push for the abolition of the slave trade. Therefore, the account is a composed of a series of examples of Newton's first hand experience of the mistreatment of African slaves. The horrors he writes of all take place at the hand of the white man (which includes himself, since he was the captain of a slave ship for some time). Therefore, he argues that the real savage in this relationship is the white man, or at least the white man who does not repent for his actions.
Perhaps Newton uses the phrase "white savage" to connect the Europeans to the slaves. In most similar narratives, the word "savage" describes a slave or an African. Therefore, Newton is calling Europeans savages here to show that Europeans and slaves have a lot in common. Newton's point is that the capability for savagery does not lie in the color of one's skin, but the evil of one's actions. He also reminds the reader that he was once a "white savage" to show that people who commit atrocities can be redeemed through religion, as he was.