The Sabbath is called menucha — rest — but Targum Pseudo-Jonathan on Exodus 35:2 makes clear it was never optional. The verse commands six days of work, then on the seventh day there shall be to you the holy Sabbath of repose before the Lord. And then the Targum sharpens the consequence: Whoever doeth work on the Sabbath day, dying he shall die by the casting of stones.
Capital punishment for Sabbath violation is the Torah's own verdict (see Numbers 15:32-36, where a man gathering sticks on Shabbat is indeed stoned by the community). The Targum preserves the severity because the Sabbath, in Jewish theology, is not merely a day off. It is the sign of the covenant (Exodus 31:17), a weekly reenactment of creation, a declaration that the world is not entirely in human hands.
The rabbis of the Talmud treated the death penalty for Shabbat violation as almost impossible to actually impose — requiring two witnesses who had specifically warned the violator, the violator proceeding anyway with full awareness, and a functioning Sanhedrin. The practical outcome, over centuries, was that almost no one was ever executed. But the principle remained: Sabbath violation is a capital-level betrayal of what Israel was chosen for.
Why so severe? Because on Shabbat a Jew declares, through a weekly act of cessation, that God is the Creator and humans are not the ultimate authors of the world. Work on Shabbat is not laziness disrupted; it is theology contradicted.
The takeaway: rest, in Judaism, is not the absence of work. It is the active assertion that the world belongs to its Maker. Keep Shabbat, and you testify. Break it, and you deny.