Why was de Tocqueville so bothered by slavery in the United States?

According to google, "Tocqueville believed that the American jury system was particularly importantin educating citizens in self-government and rule of law. He often expressed how the civil jury system was one of the most effective showcases of democracy because it connected citizens with the true spirit of the justice system."

When de Tocqueville came to the United States, he was impressed with the country because it was freer than most of the countries in Europe. So, he was especially upset that such an impressive democracy would allow a group of people to be enslaved.