Exodus 7:9-10

Exo. 7:9, 10. Moses's rod, that had been a shepherd's staff, to lead, protect, and comfort a flock of sheep, and by which Moses led and comforted Israel as a flock when cast upon the land of Egypt, became a serpent, a terrible, hurtful, and destructive creature. So Christ, that is a shepherd to his people, their protection and comfort, is destructive to unbelievers, a stone of stumbling, and a rock of offense: his salvation is poison to them through their rejection of it. They have a greater fall by the second Adam than by the first, and Christ will at last be a lion to destroy them, as that pillar of cloud of fire that gave light to the Israelites was a cloud and darkness to the Egyptians. So the word of God (which is another thing signified by the rod), which is a means of the salvation of Israel, is a sword to destroy the Egyptians.

Christ was represented by a serpent in the wilderness, because he was made sin for believers, but he will be made sin to unbelievers; he was made a curse for Israel, a serpent for them, but he will be the greatest curse to sinners, a terrible serpent to the Egyptians. So the Saviour of Israel proved the most dreadful destroyer of the Egyptians; and the word of God by Moses, which proved the salvation of his people, was their destruction. This seems to be one thing intended by this miracle, for there seems to be something threatening to the Egyptians, for the serpent had a very terrible appearance and motion, as appears by Moses's fleeing before it, when he first tried the experiment at mount Sinai. It was something threatening of the plagues that were coming. God was pleased first to threaten the Egyptians, and give them warning of approaching judgments, before he began to execute them.