Losing Free Will
The Rambam in הלכות תשובה speaks about the idea of free will. He says that this is a basic concept of Judaism. We are responsible for our decisions and actions.
However, he uses the episode in Egypt as proof that there is an exception to this rule. When someone chooses to remain evil, after having been given several harsh warnings, his free will is taken away from him.
Pharoah was hit with numerous plagues, and each time he hardened his heart and would not let the Jewish people go.
The Rambam points out that five plagues were enough of a chance for Pharoah. Because he remained so evil, his free will was taken away from him. The second five plagues were done against his will.
This is what can happen to all sinners that are steeped in their evil ways. At some point, they are treated like Pharoah, and their free will is taken away from them, and they cannot repent even if they want to. They now must be punished severely for their arrogance and stubbornness.
This is an interesting explanation that is unique to the Rambam.