According to the excpert by zamba why was the captain of the ship upset about death to slaves

Zamba Zembola, the son of a king of a small community in the Congo, was born in about 1780. When he was in his early twenties he was invited by a Captain Winton, to accompany him to America on his slave ship. After arriving in America, he was kidnapped and sold as a slave. So he was upset because he was a slave himself.