The Egyptian masses were ready to let the Jews go after the third Plague. “Don’t you know that Egypt is lost?” (Exodus 10:7), Pharaoh’s advisors told him.
However, the megalomania of a dictator makes it impossible for him to give in. According to Maimonides, wherever it says that God strengthened the heart of Pharaoh it does not mean that He took away his free will, but rather He gave him the stamina to live up to what he wanted to do! Many times we want to do something but are too weak. We buckle under the pressure. Pharaoh, too, would have buckled. However, by hardening his heart God allowed him to do what he pleased.
The masses almost always buckle before the leadership. The leader’s insanity forces the situation. Hitler led Nazi Germany to the precipice of world domination… and then down an irreversible path to its own destruction, even long after the people and most of his advisors knew it was insane to continue.
The people in Pharaoh’s time were willing to admit defeat. They saw that the events were not just natural phenomena. However, like all leaders safely ensconced in their palaces and fortresses, Pharaoh was the last to admit defeat even as the rest of the nation was suffering and facing utter destruction.
And then came the last, great plague. There was no house that did not have some dead. Death was literally at the door.
The slaying of the firstborn was the plague that finally broke the back of Pharaoh. He admitted defeat only by force in a plague that strikes home to him personally – his first born child is killed and his life is also in danger. It is not the miracle of the first-born killings that impresses him. It is the fear for his own safety that the miracle engendered that causes him to free the Jews, a decision that he almost immediately regrets. Even he realized that there was no further gain for Egypt in this contest with Moses and his God.
By then, however, it was too late.