It was Bartolomé de Las Casas.
Las Casas was a Spanish priest who gave up his encomienda (in 1514) in response to his growing concerns about the treatment of Indians in Spanish America.
One of Las Casas' most influential writings was the Brief Report on the Destruction of the Indians (1542). This recapitulation of the conquistadors' excesses was widely distributed, but was criticized then and in later years by those who thought the author had grossly exaggerated.
His writings enjoyed renewed popularity during the 19th century when they were cited by nationalists who sought independence from Spain.
Check out this brief outline of his life and labor shared by Lascasas.org.