Exodus 9

Notice that most verses say that "Pharaoh" hardened his heart and verse 12 says God hardened Pharaoh's heart. Pharaoh was always a hard-hearted man towards God. He was always selfish and prideful. He didn't really want to let the people go. Some may look at this verse and think Pharaoh is not responsible for his behavior because God made him do it. But in reality, God was not forcing Pharaoh into something that he didn't want to do. This was not a matter of Pharaoh wanting to free the people of Israel and God was stopping him. But rather, God is letting Pharaoh double down in his rejection of God's desire.