Every commentator on Matthew notes the connection between Pilate’s handwashing and the ritual of Deuteronomy 21.  Elders of a city where there is an unsolved crime - unappeased innocent blood - break the neck of a “virgin” heifer (never yoked) in a valley with living water, wash their hands over the heifer in the presence of the priests, and profess their innocence.  The heifer is killed on behalf of the city, and the water washes away the blood.  Water washes the blood from the hands of the elders, and then the living water of the valley carries the blood away.

I had been thinking that Jesus is the heifer who takes away sin, and that is true.  But in Matthew the scene is much more that Israel bears the bloodguilt of Pilate.  He follows the procedure prescribed for the elders, washing his hands “in front of” the Jewish crowd and professing his innocence.  The crowd knows what this implies: “His blood be on us and on our children.”  But there is no water to wash the blood away, and the blood clings to Jerusalem until not one stone is left on another.