Even in the 21st century, it is a sad fact the slavery is still happening in the world. Most people find this disgusting practice to be totally repugnant and immoral. Slavery was very common in biblical times and it seems strange that the biblical god did not condemn the practice of having slaves, but instead gave instructions on the keeping of slaves. The same god that condemned a man to death for simply gathering sticks on the Sabbath, a "crime" hardly in the same league as slavery.
So the question is quite simply this. Do you consider it morally acceptable to consider another human being to be your property?
If someone in the ANE couldn't pay their debts, then they could either beg, starve, or sell their future labor, and if they were able bodied, then begging wasn't an option, so slavery was an economic necessity. Furthermore, most people, especially women, were not able to survive on their own, so it was often to the advantage of someone to become a slave, where they would they would be cared for and have their basic needs met, especially if women were given the full status of becoming someone's wife. Someone who was released from slavery because they were mistreated often didn't result in long-term freedom, but in becoming someone else's slave. The major problems with slavery is when people are enslaved against their will, which the Bible forbids, and when slaves are mistreated, which the Bible also forbids.
A covenant is a very serious contract, where people would cut animals in two, stand between the halves, and say that they would become like these animals if they broke the terms of the covenant, so intentionally breaking the Sabbath was in violation of their covenant agreement, which comes with the death penalty. However, Jews do not have a history of carrying out the death penalty every time that the Law prescribed it, but rather they often imposed a fine instead. The harshness of the prescribed penalty was to show the seriousness of the offense, while the light penalty given was to show the mercy of God. In Matthew 23:23, Jesus said that justice, mercy, and faith are weightier matters of the Law, so if mercy is not evident in how someone carries out the Law, then they are not obeying it correctly. Mercy is given with the expectation that the person who committed the offense will repent and turn from continuing to commit that offense, but if someone does not seek mercy and is not repentant, then no mercy is given. We know that God is just and that He judges the heart, so the issue is not that picking up stick is such a horrible offense that anyone who does it should be killed, but rather the issue was that he was intentionally breaking the Sabbath and thereby his covenant agreement and that he was not repentant.