Christian abolitionism
The old testament in exodus details what slavery is. One fact is you could sell yourself into slavery (leviticus 25:35/39) and the word used for slave and servant is the same (abed). So we're dealing with a slightly different entity than the economical hell-hole which was chattel slavery.
So how are abed to be treated, it applies talion (eye-for-eye etc) to the slaves in Exodus 21:26-27. Talion is used in a few other verses and it doesn't all have to be stipulated to apply it (Leviticus 24:19-21, Exodus 21:22-25, Deuteronomy 19:16-21 with some of these using just eye for eye, tooth for tooth and some related dyad then saying "no mercy" which shows the sequential dyad is poetic and not literal as a simple map to talion). The middle eastern cultures used talion since hamurrabi and even in roman code but what makes Exodus 21:26-27 special is it says the slave is to go free. Exodus 21:23-25 goes over how extensive talion is and what "no mercy" means, "23 But if there is serious injury, you are to take life for life, 24 eye for eye, tooth for tooth, hand for hand, foot for foot, 25 burn for burn, wound for wound, bruise for bruise." So the application of talion to slaves in the succeeding two verses says to not hurt your abed or they go free.
Slavery is hardly a surprise to anyone in that it happened in history but what is surprising to some are the preceding verses in Exodus 21:20-21 where it says, "20 “Anyone who beats their male or female slave with a rod must be punished if the slave dies as a direct result, 21 but they are not to be punished if the slave recovers after a day or two, since the slave is their property." These verses without the succeeding ones sounds like it allows beating of abed but it simply says the abed owner has no consequence unless they hurt the abed a lot and I'm not entirely sure what the reason was for that but the abed is still let go. I imagine these were applied so talion didn't apply backwards (enslavement-for-enslavement). We see this "owner-does-not-receive-punishment-unless-they-went-too-far" applied to bulls in Exodus 21:28-32 where there is a punishment and line an owner of a bull can't cross otherwise he gets punished. There were also verses about not being cruel to animals as part of the noahide covenant (which influenced the ten commandments).
The basis for all this is in Leviticus 25:42 where he says, "Because the Israelites are my servants, whom I brought out of Egypt, they must not be sold as slaves." It uses the same word abed in the original hebrew (but conjugated/declined differently to represent case and plurality) but the point of this covenant was that they were freed from Egypt by YHWH who they were enslaved to and that was the foundation of their ethics etc.