“I will harden Pharaoh’s heart.”

When God hardens Pharaoh’s heart, he does not cause Pharaoh to sin, but punishes Pharaoh’s sins—and not Pharaoh’s only but Egypt’s sins: “But Pharaoh shall not hearken unto you, that I may lay my hand upon Egypt.” So a punishment for sin is more sin. This does not mean that a sinner cannot help sinning, but that he is more and more inclined to sin—is more stubborn in his sinning. “Let me sin once more, then I'll repent” is a delusion, all the more so when it seems to be true.