I’d like to convey two answers offered by one of the greatest Torah scholars of the early twentieth century, Rav Shlomo Yosef Zevin ZT”L, in his classic “La-torah V’La-Moadim”.

1) We are all born with innate characteristics, or Middot. It is up to us to decide how to apply them. For example, someone who is born with a tendency toward spilling blood, can choose to be a murderer, or a Mohel or Shochet. God instilled Pharaoh with a strong tendency towards stubbornness. He hardened his heart. Pharaoh had the free will to apply his powerful stubbornness to either stand firm against his emotional desire to maintain power and lordship over an innocent nation, or to stand firm against the plagues sent to convince him to release that same nation. Pharaoh freely chose to direct his hardened heart against the Jews.

2) Doing something because you are forced to is not a sign of freedom. It is a sign of powerlessness. If Pharaoh had released the Jews solely as a reaction to the horror of the plagues, he would not have been acting out of his own free will. Therefore, God hardened his heart, in order to neutralize his fear of the plagues, and allow him to decide whether to release the Jews solely based on his own free will and moral character. By hardening Pharaoh’s heart, God actually allowed him to exercise his free will.