John 18:31

Verse 31. It is not lawful for us to put any man to death] They might have judged Jesus according to their law, as Pilate bade them do; but they could only excommunicate or scourge him. They might have voted him worthy of death; but they could not put him to death, if any thing of a secular nature were charged against him. The power of life and death was in all probability taken from the Jews when Archelaus, king of Judea, was banished to Vienna, and Judea was made a Roman province; and this happened more than fifty years before the destruction of Jerusalem. But the Romans suffered Herod, mentioned Acts 12:1, c., to exercise the power of life and death during his reign. See much on this point in Calmet and Pearce. After all, I think it probable that, though the power of life and death was taken away from the Jews, as far as it concerned affairs of state, yet it was continued to them in matters which were wholly of an ecclesiastical nature and that they only applied thus to Pilate to persuade him that they were proceeding against Christ as an enemy of the state, and not as a transgressor of their own peculiar laws and customs. Hence, though they assert that he should die according to their law, because he made himself the Son of God, John 19:7, yet they lay peculiar stress on his being an enemy to the Roman government; and, when they found Pilate disposed to let him go, they asserted that if he did he was not Caesar's friend, John 18:12. It was this that intimidated Pilate, and induced him to give him up, that they might crucify him. How they came to lose this power is accounted for in a different manner by Dr. Lightfoot. His observations are very curious, and are subjoined to the end of this chapter.

See Clarke on Joh 18:40