Answer:
Equiano wanted to show the white man's cruelty through his narrative. He included a description of the flogging of a white man to show that if a white man could be so cruel towards a member of his own race, how would he treat a member of the black race, which at the time he considered inferior. He started fearing for his own life even more at this point.
"One white man in particular I saw, when we were permitted to be on deck, flogged so unmercifully with a large rope near the foremast, that he died in consequence of it; and they tossed him over the side as they would have done a brute. This made me fear these people the more; and I expected nothing less than to be treated in the same manner."