A.T. Robertson Commentary John 19:7

"The Jews answered him, We have a law, and by that law he ought to die, because he made himself the Son of God." — John 19:7 (ASV)

Because he made himself the Son of God (οτ υιον θεου εαυτον εποιησεν). Here at last the Sanhedrin give the real ground for their hostility to Jesus, one of long standing for probably three years (John 5:18) and the one on which the Sanhedrin voted the condemnation of Jesus (Matthew 27:23–66), but even now they do not mention their own decision to Pilate, for they had no legal right to vote Christ's death before Pilate's consent which they now have secured.