There is no anger in Abimelech's voice, but there is pain. The Targum Pseudo-Jonathan records the king calling Isaac and saying, "Nevertheless she is thy wife. Why hast thou said, She is my sister?" (Genesis 26:9).
Isaac's answer is the same answer his father gave two kings before. "Because I said in my heart, Lest they kill me on her account."
The king who knows better
Pseudo-Jonathan treats Abimelech more gently than a modern reader might expect. This is the second Abimelech — perhaps a dynasty, not one man — and he is not a tyrant. He asks. He probes. When he discovers the truth, he is not violent; he is wounded. The Targum is showing us a gentile king who fears God enough to be disturbed by Isaac's deception.
Isaac's defense — lest they kill me — is not a lie. The Targum records it as the honest fear of a man in a land where foreigners had no legal standing. But it is also not the whole truth. The covenant that should have protected Isaac was not the sword of Abimelech. It was the Word of the Holy One, who had just promised "My Word shall be for thy help" (Genesis 26:3).
The takeaway
Fear is not a sin. But fear that forgets the promise is a forgetting that stains. The Targum preserves the exchange to teach that even the patriarchs needed to be reminded of what they had been told the night before. We are not stronger than they were. We too need the reminder.