Why did G-D need to harden Pharaoh's heart? Why was there a need for the Ten Plagues? Why couldn't G-D bring about the Exodus by having Pharaoh come to the realization that it was to his benefit to get rid of his Hebrew slaves? Hadn't Pharaoh expressed his fears that in the event of war, the Hebrew nation would join our enemies and wage war against us (Shemot 1:10)? If he was so afraid that the Jews would be collaborators against him, and by their miraculous population explosion soon outnumber the Egyptians, why didn't he just let them go? 

Rav Shlomo Kluger explains that it would have been a disaster if the Exodus from Egypt would have come about in this way, with Pharaoh playing the role of the Great Emancipator. Instead of Klal Yisrael being free from Pharaoh, this would have created a situation where the Jewish nation would be eternally indebted to Pharaoh for giving us independence by permitting us to leave Egypt. 

The trait of HAKARAT HATOV (gratitude) is deeply engrained in the heart and soul of our nation and is the essence of many Mitzvot in the Torah. Even a kindness done to our ancestors many centuries ago creates a debt of gratitude that we must continue to honor and not allow to diminish with the passage of time. 

The very purpose of the exodus of Egypt was that "I shall take you to Me for a people and I shall be a G-D to you, you shall know that I am Hashem your G-D, Who takes you out from under the burdens of Egypt" (Shemot 6:7). The only way the Jewish Nation could show its total loyalty and express its boundless thanks to G-D for taking us out of Egypt was if we had no reason to show any gratitude to Pharaoh. Our gratitude must be solely for G-D alone.

Therefore, G-D had to bring out the very worst in the Egyptian King, and continually harden his heart to refuse Moshe's request to let us go free. This was achieved by afflicting Pharaoh and his nation with the Ten Plagues. Despite the devastation the plagues caused, pharaoh stubbornly refused to give in until the death of the first born finally caused him to let the Jews go.

The Exodus from Egypt is what made our people into the Nation of G-D. Numerous Mitzvot that we perform are a memorial of the exodus from Egypt, and the miracles that G-D performed during the Exodus is the basis of our EMUNAH in Him.

Therefore, if Israel would in any way be indebted to Pharaoh, that would detract from the total loyalty and gratitude we owe to G-D for taking us out of the Egyptian bondage. Thus, G-D had to remove Pharaoh's Free Will for our eternal benefit.