By the eighth plague, the Torah's language has shifted. Before, it was Pharaoh who hardened his own heart. Now, the Lord takes a share of the responsibility.
"The Lord spake to Mosheh, Go in unto Pharoh; for I have made strong the design of his heart, and the design of the heart of his servants, to set these My signs among them" (Targum Pseudo-Jonathan on Exodus 10:1).
The Aramaic paraphrase, preserved in the Targum attributed to Yonatan ben Uzziel, uses the phrase takif yat libeh — I have strengthened his heart. The sages of the Talmud and midrash wrestled with this line for generations. If God hardens Pharaoh's heart, is Pharaoh still responsible for his refusal?
The classical answer runs like this: for the first five plagues, Pharaoh hardened his own heart freely. He was exercising ordinary human stubbornness. But by the sixth, that stubbornness had become so deep that it became his nature. At that point, the Holy One simply allowed Pharaoh to continue being the person he had already chosen to be. God did not invent Pharaoh's cruelty. He confirmed it.
And the reason? L'shavaa atayai bein'hon — to set My signs among them. The remaining plagues would not be for Pharaoh's conversion. They would be for Israel's memory. The Maggid teaches: sometimes the Holy One lets a wicked man finish becoming himself so that the world can learn from watching him fall.