Yes, God.

God told Moses to say to Pharoah to let my son go. Israel is my son, my first born. Tell Pharoah to let my son go so he may serve Me. God was talking about the Israelites--God's chosen people whom Pharaoh kept under hard bondage. God told Moses to say that if you refuse to let my son go I will slay your son who is your first born. (Exodus 4:23) All the firstborn in Egypt shall die, from Pharaoh's first born, the first born of the maidservants and all the first born of the animals. But against the children of Israel there shall no harm come to man or beast.

This all sounds so familiar. Didn't the Pharaoh do this to God's people? Didn't Pharaoh command his people to kill all the male babies? Didn't Moses, himself, narrowly escape the Pharaoh's edict to kill all the male babies?

And it came to pass, at midnight, that the Lord smote all the firstborn in Egypt, from the throne to the first born in the dungeon to the first born of cattle (Exodus 12:29).

God had already told Moses, in advance of Moses returning to Egypt, that Pharaoh was not going to let His people go. Why, after Moses turned his rod into a snake and turned the water into blood right in front of Pharaoh's face would the Pharaoh refuse to let the Israelites leave? The Lord told Moses that I want you to go to Pharaoh, tell him to let God's people go so that they may serve Him; have your rod in your hand, show the signs and wonders to Pharaoh, and he is not going to let My people go, because I am going to harden Pharaoh's heart.