“Harden” means refusing to repent. God allowed Pharaoh’s heart to be hardened. It is sin that hardened a sinner’s heart against God. God did not make Pharaoh against God Himself.
Likewise, today if a sinner goes to hell, it is because of his sins; but if a sinner repents, it is because of God’s grace.
1 Samuel 6:6, “Why should you harden your hearts as the Egyptians and Pharaoh hardened their hearts?”