Adam Clarke on Matthew 27:24

Pilate - took water, and washed his hands - Thus signifying his innocence. It was a custom among the Hebrews, Greeks, and Latins, to wash the hands in token of innocence, and to show that they were pure from any imputed guilt. In case of an undiscovered murder, the elders of that city which was nearest to the place where the dead body was found, were required by the law, Deu 21:1-10, to wash their hands over the victim which was offered to expiate the crime, and thus make public protestation of their own innocence. David says, I will wash my hands in innocence, so shall I compass thine altar, Psa 26:6. As Pilate knew Christ was innocent, he should have prevented his death: he had the armed force at his command, and should have dispersed this infamous mob. Had he been charged with countenancing a seditious person, he could have easily cleared himself, had the matter been brought before the emperor. He, therefore, was inexcusable.