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

Perhaps we can start by asking a few questions to get a broader understanding of what might be going on here.

What is the Sabbath?

The Sabbath is a day of rest. God established the pattern of the week at creation, blessing the seventh day and making it holy "because on it God rested from all his work that he had done in creation." (Gen 2:3) This is picked up again in the giving of the Ten Words (or commandments) in Exodus 20, "Six days you shall labor, and do all your work, but the seventh day is a Sabbath to the LORD your God. On it you shall not do any work, you, or your son, or your daughter, your male servant, or your female servant, or your livestock, or the sojourner who is within your gates. For in six days the LORD made heaven and earth, the sea, and all that is in them, and rested on the seventh day." It is a day not just of rest for yourself, but also for all those over whom you have charge. It is a day of rest and of giving rest. Doing work on the Sabbath is clearly forbidden and is punishable by death (Exodus 31:14-15, 35:2). However, as Jesus makes clear later, this prohibition does not prohibit doing good or helping someone. If your ox falls in a ditch, you can get it out (Luke 14:5).

The Sabbath is a feast day. Leviticus 23 outlines the major feasts of the year, beginning with the weekly Sabbath. "These are the appointed feasts of the LORD that you shall proclaim as holy convocations; they are my appointed feasts. Six days shall work be done, but on the seventh day is a Sabbath of solemn rest, a holy convocation. You shall do no work. It is a Sabbath to the LORD in all your dwelling places."

The Sabbath is a day of gathering to worship. As noted above, the Sabbath day is a day of "holy con-vocation," a calling-together of the people of God. The people could bring an offering any day of the week, but the Sabbath was set aside for the particular purpose of coming together as a body to worship God.

What about gathering sticks?

It is clear from even a cursory reading of the passage that this guy has violated the first principle of the Sabbath: he is doing work. This is forbidden and deserving of death. But, it can be argued, other seemingly similar examples can be put forward that didn't end up with the death penalty. God told the people not to gather manna on the Sabbath, but they tried anyway. Now, they weren't actually able to, because it wasn't available, but they set out to do work that was forbidden and all that was said was that they should go back to their tents (Exodus 16:29). It seems there must be more going on here. Why is the man gathering sticks? To fuel his fire back at his tent.

Exodus 35 sheds a little more light on this. "Moses assembled all the congregation of the people of Israel and said to them, 'These are the things that the LORD has commanded you to do. Six days work shall be done, but on the seventh day you shall have a Sabbath of solemn rest, holy to the LORD. Whoever does any work on it shall be put to death. You shall kindle no fire in all your dwelling places on the Sabbath day.'" This ties kindling a fire on the Sabbath directly to the general prohibition against work on the Sabbath and with the entirety of the commands that God had given His people. What is it about kindling a fire on the Sabbath that would be so bad?

Numbers 28 goes over the regular offerings that would be offered daily, weekly, monthly, and yearly. The daily offerings were two male lambs a year old, one in the morning and one in the evening, with the attendant flour, oil, and drink offering. God calls this "my offering, my food for my food offerings, my pleasing aroma." This echoes the same language used for the description of the offerings in Leviticus (cf. 1:9, 13, 17). Every Sabbath, this same offering was given as a Sabbath offering in addition to the regular daily offering. These are the ascension (or 'burnt') offerings which were completely burned up on the altar as a food offering. The fire on the altar was lit by God (Leviticus 9:24) and represented His presence with His people (cf. Numbers 9:15) On the Sabbath, it was built up twice as much as normal for the doubled offering.

Putting it together

Seeing the Sabbath as a day of rest and of giving rest, a day of feasting, and a day of con-vocation helps make some sense of what is going on. God as host calls His people together to the feast. The shared meal makes them one body (cf. 1 Corinthians 10:17) as they gather to the fire of His presence, His hearth fire, which is doubled for the day. This man's sin is not merely gathering sticks. If he kindles his own fire, he is not participating in the community, the body. But you don't stoke up a fire on a feast day just for yourself. This, in addition to dividing the community, places him over and against God. For him to kindle his own fire on the Sabbath is to kindle a rival fire. He is setting himself up as a rival host calling a rival gathering on the Lord's feast day. Now the seriousness of the crime and the proclamation of the death penalty becomes more clear.