Actually, slavery in the Torah is demonstrably better than prison. In the former the thief or bankrupt gets to learn how to stand on his own two feet. In the latter he learns how to be a bigger crook. If he is emotionally unable to tolerate freedom he need not commit another crime to go back to servitude. While there was also the eved Kenani, who was not a full Jew, he had many rights, such as Shabbat and Yom Tov. If his master injured him or needed him for a mitzva (e.g. completing a minyan) he went free and became a full Jew.
