Pharaoh seemed to understand that he had reached a point of no return. The plagues had shown him the power of the Lord. The only proper response was to submit.
Instead, he chose another path. If he would not submit, then he would try to silence the messenger. So he decided to threaten Moses rather than obey God.
This pattern appears again later in Scripture. During the life and ministry of Jesus, some religious leaders refused to accept what his works clearly showed. Instead of believing, they chose to kill him (see John 11:45–53).