Moses's reply to Pharaoh's death-threat is magnificently calm — and the Targum reveals why.
"Thou hast spoken fairly. While I was dwelling in Midian, it was told me in a word from before the Lord, that the men who had sought to kill me had fallen from their means, and were reckoned with the dead. At the end there will be no mercy upon thee; but I will pray, and the plague shall be restrained from thee. And now I will see thy face no more" (Targum Pseudo-Jonathan on Exodus 10:29).
The Aramaic paraphrase, preserved in the Targum attributed to Yonatan ben Uzziel, adds a detail that unlocks the whole scene. Moses already knows that the men who once sought his life are either dead or powerless. The Lord had told him so in Midian, before the burning bush, before the return to Egypt. So when Pharaoh threatens to release assassins, Moses is unmoved — those assassins cannot harm him. Heaven has already told him so.
And then Moses delivers his own warning. B'sofa lo yehei rachamei alach — at the end, there will be no mercy upon you. He is speaking of the tenth plague, which is now only hours away. And he says: I will pray for your people, I will even contain the plague where I can, but there is a limit, and you have crossed it.
V'lo osif l'mechzei sever apach — I will see your face no more. Moses is done negotiating. The Maggid teaches: there is a moment when the prophet stops speaking and God takes over. Moses walks out of that palace knowing he will never return. The next conversation between heaven and Egypt will not use words. It will use angels.