Let's look first at the case of the foreign bondman: the verses #1689769 quotes are followed by Exodus 21:26-27:
"An owner who hits a male or female slave in the eye and destroys it must let the slave go free to compensate for the eye. And an owner who knocks out the tooth of a male or female slave must let the slave go free to compensate for the tooth."
It is wrong to say that the foreign bondservant/slave lives under conditions where they maybe beaten to death at any moment. Firstly, if they run away, those to whom they have run may not return them (Deuteronomy 23:16-17). Likewise, it is forbidden to make them work on the Sabbath (Exodus 20:10); Exodus 23:2 requires them to be given sufficient rest. Given that non-Hebrew bondservants are seen as an extension of the Jewish community, it is forbidden to sell them to non-Jews. So even if Dan Marvin lived in Biblical times, none of his musings would apply to him unless, of course, he were a non-Jewish bondservant/slave.
What #1689769 quotes in verses 20-21 relates to the law of damages exacted by the state against the owner. The relationship of the master to the bondservant/slave is very different; if the master so much as breaks the tooth (and verses 24-25 indicate that "tooth" stands in for anything down to a bruise) of a slave, the slave must go free. Not only does the slave go free but, as Maimonides notes in the Mishneh Torah, he or she immediately gains the status of a Jew, on an equal footing with his or her former owner.
As for Jewish slaves, simply "paying off a debt" is not good enough; it has to be in a condition where the person is destitute. However, if a person is destitute, Jews are obligated to give that person alms so that they may support themselves; altogether, the Bible strongly disapproves of the institution - see Jeremiah 34:8-24. The only condition where a Jew becomes a bonded servant without their permission is if that person is a thief (Exodus 22:2). Any Jewish slave must not be made to work indefinitely on anything, nor overworked (Leviticus 25: 39, 43), or given any humiliating task. They may redeem themselves; all slaves are released on a jubilee year. All slaves, whenever they are released, are given a parting gift of thirty shekels.
All of this, of course, ignores the subsequent development of Judaism which put so many bans and conditions around the holding of slaves or bondservants, Jewish or not, that it effectively abolished slavery about a thousand years before abolitionism became popular in Christian Europe and America.