Why did God kill a man for picking up sticks on the Sabbath?

> Now while the sons of Israel were in the wilderness, they found a man gathering wood on the Sabbath day. Those who found him gathering wood brought him to Moses and Aaron and to all the congregation; and they put him in custody because it had not been declared what should be done to him. **Then the Lord said to Moses, “The man shall surely be put to death; all the congregation shall stone him with stones outside the camp.”** So all the congregation brought him outside the camp and stoned him to death with stones, just as the Lord had commanded Moses. **- Numbers 15: 32-36 (NASB)** Why did God order the execution (and a brutal execution, at that) of a person for picking up sticks on the Sabbath? Surely what this man did was nothing worse than what Jesus' disciples did when they picked heads of grain on the Sabbath: > And it happened that He was passing through the grainfields on the Sabbath, and His disciples began to make their way along while picking the heads of grain. The Pharisees were saying to Him, “Look, why are they doing what is not lawful on the Sabbath?” > > ... > > Jesus said to them, “The Sabbath was made for man, and not man for the Sabbath. **- Mark 2:23-27** Such a harsh punishment doesn't seem necessary if the Sabbath truly was made for man, and not man for the Sabbath. Also, how does this align with the justice of God? God forbids the punishment of those who beat their slaves: > If a man strikes his male or female slave with a rod and he dies at his hand, he shall be punished. If, however, he survives a day or two, no vengeance shall be taken; for he is his property. **- Exodus 21:20-21** ... yet He requires those who pick up sticks on a certain day of the week to be executed in the most unimaginably painful and cruel way. In our 21st century minds, it seems that justice would demand that the punishments for these two crimes be switched. What was God's reason for having such an inverted justice system?