Dalzel studied medicine at Edinburgh, and served in the Royal Navy as a surgeon during the Seven Years’ War (1756–63). Discharged in 1763, he accepted a position as a surgeon in the Company of Merchants Trading to Africa, and was stationed at Anomabu on the Gold Coast. While in West Africa he began slave trading. Considering the horrifically inhumane aspects of slave trading, it is possible that Dalzel was the only physician who actually profited directly from the trade, and was motivated to write a book defending it.
From 1767 to 1770 Dalzel was director of the British fort at Ouida. He then concentrated on slave trading until he was declared bankrupt in 1778. Dalzel was active in the campaign opposing abolition. He argued in this book that the raiding of Dahomean villages for slaves was "saving them from the great evil of being human sacrifices." Digital facsimile from Google Books at this link.