While it is hard to say what any historical figure believed in their heart of hearts, the answer would be yes based on his subsequent statements. Las Casas, referring to himself in the third person wrote (emphasis added): "the cleric, many years later, regretted the advice he gave the king on this matter—he judged himself culpable through inadvertence—when he saw proven that the enslavement of blacks was every bit as unjust as that of the Indians. It was not, in any case, a good solution he had proposed, that blacks be brought in so Indians could be freed. And this even he thought that the blacks had been justly enslaved. He was not certain that his ignorance and his good intentions would excuse him before the judgment of God."

Quote from Clayton, L. "Bartolomé de las Casas and the African Slave Trade" History Compass vol. 7, no. 6, 2009.