Exodus 8:15 (NKJV), “But when Pharaoh saw that there was relief, he hardened his heart and did not heed them, as the LORD had said.” It was the Lord’s mercy which caused Pharaoh to harden his own heart. Pharaoh didn’t want to lose the Israelites’ slave labor. When relief came, he thought he could do as he wished. Pharaoh did not care that both the Egyptians and Israelites suffered in the plagues. He had a hard, greedy heart.
