How מכה after מכה, פרעה could just go and still deny בנ"י the right to leave is preposterous. And maybe you’ll say ה' hardened his heart- but this is only after the last five מכות; the first מכות, פרעה hardened his own heart. He was wicked, but how could someone endure so much? Please bring מקורות.
This is a complex question that is dealt with by a number of commentators.
Did Pharaoh Have Free Will? quotes Ramban as saying
Nachmanides offers an answer which is as profound as it is astoundingly simple. He argues that had G‑d refrained from hardening Pharaoh's heart, he would have then been deprived of the ability to make a coherent and true choice. Indeed, the plagues would have compelled him to let the Israelites go — an option he most certainly would not have chosen in the absence of G‑d’s strong hand
Rambam on the other hand seems to say that after insisting on not letting them go, he was punished by losing his free will. Since he insisted on continuing to do evil after all the chances he was given, he could no longer do teshuvah.
Rav Hirsch actually gives a third explanation. He says that Hashem carefully limited the makos which allowed Par'o to rationalize that there was a "power" that Hashem could not conquer and was protecting whatever had not been affected by that particular maka. In actuality, Hashem had done this to allow the Mitzri'im to do teshuvah and let Bnai Yisrael go without having to totally destroy Mitzraim. An example of this is barad which destroyed the crops that were ready for harvest (spelt) but let the partially grown crops survive (wheat - since the wind cause them to bow down so the hail would not destroy them). That is why Hashem said that the crops that the hail left would be eaten by the Arbeh.
Rabbi Sorotzkin (Oznaim Latorah) says that the reason it says Par'o and his servants is because whenever the servants started to weaken, Par'o would strengthen them. Whenever Par'o started to weaken, the servants strengthened him. The y would also remind Par'o that the previous Par'o had been deposed when he first objected to oppressing the Bnai Yisrael and only regained the throne after he agreed to enslave them (Medrash Hagadol).
Hardened Hearts: Some Explanations gives various explanations and discusses problems with them.
Most explanations are based on the Ramban and Rambam.
Elements of the “modest” claim are found in Moses Nachmanides’ and Obadiah Sforno’s commentaries to Exodus 7:3, and in Joseph Albo’s Book of Roots, IV:25. But all these philosophers put forth the modest claim in the context of a wider strategy, rather than present it in isolation as I have just done. And this is understandable, for the approach seems to provide at best a defense against the charge of free will deprivation and repentance prevention, not an explanation of why God hardens. In addition, more must be said if we are to explain why Pharaoh is responsible for the hardened act.
According to what I call “the bold claim,” Pharaoh’s act of keeping the Israelites enslaved is in truth free, despite God’s intervention. When God “hardens” Pharaoh’s heart, this means merely that he strengthens Pharaoh’s heart, giving him the fortitude not to let the plagues automatically dictate a decision to release the Israelites.
Thanks to the hardening, the king now has a choice: whether to release the Israelites or to keep them enslaved. Two possibilities are open to him, whereas without the hardening he had but one (to release the Israelites). The existence of this choice suggests he is responsible for his (freely chosen) hardened act, and that he has a possibility to repent. (See Albo, Book of Roots, IV:25)
A different version of the bold claim runs as follows: by increasing the king’s willpower (by weakening certain desires and/or strengthening others), God, de facto, is allowing Pharaoh to act in accordance with his already formed character, and thus to act freely.
According to yet another view, God hardens the agent’s heart as a means of punishing him. Specifically, the hardened agent is thereby deprived of three great goods: (a) free will, along with (b) the potential to act rightly, and (c) the chance to repent. He is not punished further for the hardened act.
But even granted the heinous character of figures like Pharaoh, some thinkers have been troubled by the implication that God actively shuts the gates of repentance to some people. (See, for example, Arama, Akedat Yitzchak, Exodus, chapter 36; cf. Maimonides, Eight Chapters, Chapter 8, and Mishneh Torah, Laws of Repentance Chapter 6.)