Adam Clarke on Exodus 31:14

Every one that defileth it - By any kind of idolatrous or profane worship.

Shall surely be put to death - The magistrates shall examine into the business, and if the accused be found guilty, he shall be stoned to death.

Shall be cut off - Because that person who could so far contemn the Sabbath, which was a sign to them of the rest which remained for the people of God, was of course an infidel, and should be cut off from all the privileges and expectations of an Israelite.