Exodus 7:17 - Matthew Poole's English Annotations on the Holy Bible

Because thou saidst, Who is the Lord? and, I know not the Lord, Exodus 5:2, thou shalt know him experimentally, and to thy cost. Behold, I will smite, viz. by Aaron's hand, who shall do it by my command and direction. Thus Pilate is said to give Christ's body to Joseph, Mark 15:45, because he commanded it to be delivered by others to him. The same action is ascribed to the principal and instrumental cause. The river Nile, which was one of their principal gods; and therefore it was inexcusable in them, that they would not renounce those feeble gods, which were unable to help not only their worshippers, but even themselves, nor embrace the service and commands of that God whose almighty power they saw and felt. They shall be turned to blood, which was a very grievous plague to them; both because it was an eternal dishonour to their religion, and because from hence they had both their drink, Deuteronomy 11:10,11 Jer 2:18, and their meat, Numbers 11:5; for greater and lesser cattle they would not eat, Exodus 8:26. And it was a very proper punishment for them, who had made that river an instrument for the execution of their bloody design against the Israelitish infants, Exodus 1:22.