The Targum Pseudo-Jonathan captures Jethro's theological breakthrough in one line: "Now have I known that the Lord is stronger than all powers; for by the very thing by which the Mizraee wickedly would have punished Israel by (drowning them in) the sea, upon themselves came the punishment, in being punished in the sea" (Exodus 18:11).
This is the doctrine of middah k'neged middah — measure for measure — articulated by the first convert in the Torah. Pharaoh had decreed in Exodus 1:22 that every Hebrew baby boy be thrown into the Nile. The Egyptians had planned to drown Israel. So they drowned.
Jethro's phrasing is precise. He does not say simply that Egypt was defeated. He says they were punished by the very thing they intended to use. The instrument of cruelty became the instrument of justice. The Aramaic underlines it: "by the very thing by which... upon themselves came the punishment."
The rabbis draw out this theme in countless midrashim — Haman hanged on his own gallows, the sons of Jacob eating grain in the same Egypt where they sold Joseph, Samson bringing down the pillars he was chained to. Jethro is the one who first names the pattern.
The takeaway: evil is often undone by its own mechanics. The trap you build for others is, in the moral architecture of God's world, the trap most likely to catch you.