Exodus 9:12: God Ups the Ante

When Israel was in Egypt's Land, Let my people go.
Oppressed so hard they could not stand, Let my people go.
"Thus saith the Lord," bold Moses said, "Let my people go:
If not I'll smite you first born dead, Let my people go."
Go down, Moses,Way down in Egyptland;
Tell old Pharaoh "Let my people go." - Old Negro Spiritual

This song doesn't say that Pharaoh was a gambling man, but he was.  After the Egyptians cattle died, a plague of boils, and a plague of hail, he finally agrees to let the Israelites go, until it stops hailing and  then he changes his mind.  Moses and Aaron go back to Pharaoh and deliver God's message that there will be a plague of locusts if the people can't leave.  By this time, even Pharaoh's servants are behind Moses.  Then Pharaoh’s servants said to him, “How long shall this man be a snare to us? Let the men go, that they may serve the LORD their God. Do you not yet know that Egypt is destroyed?”   Exodus 10:7 NKJV

And the bargaining continues...Pharaoh says he will let only the men go, but reneges and there is a plague of locusts.  But when the locusts are blown away, so is Pharaoh's promise.  After the plague of darkness, he give permission for the people to leave, but not the flocks.  By this time, it wasn't just God and Moses who were getting tired of Pharaoh's shenanigans, even his own people were frustrated and more than willing for the Israelites to leave.  And the LORD said to Moses, “I will bring one more plague on Pharaoh and on Egypt. Afterward he will let you go from here. When he lets you go, he will surely drive you out of here altogether.  Speak now in the hearing of the people, and let every man ask from his neighbor and every woman from her neighbor, articles of silver and articles of gold.”  And the LORD gave the people favor in the sight of the Egyptians. Moreover the man Moses was very great in the land of Egypt, in the sight of Pharaoh’s servants and in the sight of the people.  Exodus 11:1-3 NKJV

And then, the day for bargaining was past.  That was it, there would be no more.  There would be no more first born in Egypt.  The Israelite first born would be saved by the blood and the rest would be lost.  The die was cast.