וְעַכְשָׁיו נֶהֶפְכוּ לִרְדֹף אַחֲרֵיהֶם בִּשְׁבִיל מָמוֹנָם שֶׁהִשְׁאִילוּם:
AND IT WAS TOLD THE KING OF EGYPT — Now, however, they (their hearts) were changed, prompting them to pursue them, because of the property that they had handed over to them (cf. Mekhilta d'Rabbi Yishmael 14:5:3).
(1) I will harden Pharaoh’s heart. Pharaoh had been fearful of them ever since the striking of the first born, as evidenced by his request, “bless me too” (12:32), and did not intend on pursuing them even if they tried to flee. Therefore God informed Moshe that He would harden Pharaoh’s heart to chase after them. He repeats this in verse 17 because after the Egyptians saw the sea split and B’nei Yisrael pass through on dry land they should have been too terrified to continue. God, however, caused them to lose their senses and hardened their hearts to enter the sea.
From the Egyptian standpoint,... the new king would only allude to the Eighteenth Dynasty ruler Ahmose I who succeeded in expelling the Hyksos after a protracted period of time... With the foreign rulers dislodged from Avaris and their control over the Delta and parts of middle Egypt, the Hebrews would have found themselves in the uneviable position of being too closely associated with Semitic rulers to be ignored... Moreover, an association of the Hebrews with the fellow Semites who had ruled Lower Egypt might explain the hostile treatment and the paranoid Pharaoh's statement, "if war befall us, they will join our enemies (the recently departed Hyksos) and fight against us" (Ex 1:10). - J.K. Hoffman, 1997, NY
Pharaoh assumed that any request to go to the desert to worship God (or to emigrate to Canaan) was simply a "ploy" to hide the first step of a planned insurgency. - Rabbi Menachem Leiptag, 21st cent. Israel
Paranoia is the irrational and persistent feeling that people are 'out to get you'.
There are three Hebrew words for the “hardening” of Pharaoh’s heart: kashah – to become hard; kaved – to become heavy; chazak – to become strong or overpowering. These are the characteristics of the oppression of tyranny. (Samson Raphael Hirsch, 20 cent.)
There are three general meanings of "God stiffened Pharaoh's heart":
1. God made Pharaoh stubborn right from the start so he would be foolish in order to publicly defeat him and show the world (including both the Egyptians, the Israelites, and you the reader) that there is only one God. Each of the Ten Plagues kills off a deity of the Egyptian pantheon, and the splitting of the sea was God's cosmic victory. In addition, the genocidal Pharaoh was so wicked he didn't deserve the opportunity for repentance. If he had lived a life of sin and then repented at the end without being punished, it would have been unjust. (Rabbeinu Bachya)
2. Pharaoh may have started off with free will, but Pharaoh became so entrenched in his own self-idolization that he lost his free will. "God hardening his heart" means he had crossed the point of no return. Melech mitzraim = "the kind of constriction." (Exodus Rabbah 13:3, Rashi, Rabbi Nachman of Breslov)
3. God bolstered Pharaoh against the pain ("strengthened his heart"), so Pharaoh would have free will and make his own choices. (Saadia, Albo, Sforno)
4. An additional, fourth explanation focuses on the idea that God was making Pharaoh's heart "heavy": In Egyptian lore, after death, Osiris would take the Pharaoh's heart and weigh it against "the feather of truth." Only a light heart would be permitted to journey on into the afterlife. There may be an echo of this belief system in the Torah, that God is weighing down Pharaoh's heart because of his sins.