זוהר בראשית — דף רלט ע”א

Pharaoh hardened his heart, and the Holy One then hardened it further, for the measure-for-measure principle applies in this direction as well. Each time Pharaoh hardened his heart, a door of repentance closed. Pharaoh initially had an opening for teshuva but chose wickedness. The lesson is that free will is granted to everyone, even Pharaoh. The secret is that freedom of choice belongs to every human being, and the consequences of those choices are inexorable.