Answer:
1. He believed they were Indian, which is actually the reason that name was given to natives in all of the Americas. Since Columbus tried to sail west to reach India, when he reached the Americas he assumed he was in India.
2. He believed all natives were the same. Back then, the only people most Europeans recognized Christians and pagans. Since the natives were not Christian they were just tossed into one big group as "Indians", with no difference between their religions, language, or beliefs.
3. He believed they had no soul, and as such they were less than human. Again, because Columbus was a Christian, he believed that because indigenous people didn't know God and refused to accept Christianity, that they had no soul and could not be saved, so he treated them like animals and sold them into slavery.