Regarding Eved K'naani, the Torah itself gives the reason for the
institution. Noach recognizes that Canaan is of debased "cursed"
character and pronounces this reality. Slavery to those of superior
character is intended as a form of moral and educational rehabilitation.
This is discussed by Rabbi Chaim Eisen in Jewish Thought journal years
ago.
The issue of slavery in later times and regarding people who are not
necessarily Canaan is a complex subject for which I have no particular
insight. Understandably the issue disturbs people.
