Charles Ellicott Commentary John 18:31

"Pilate therefore said unto them, Take him yourselves, and judge him according to your law. The Jews said unto him, It is not lawful for us to put any man to death:" — John 18:31 (ASV)

Take ye him, and judge him according to your law.—Pilate takes them at their word. They claim the judicial right; let them exercise it. Their law gave them power to punish, but not the right of capital punishment. If they claim that the matter is wholly within their own power of judgment, then the sentence must also be limited to their own power. He can only execute a sentence which is pronounced by himself after formal trial.

It is not lawful for us to put any man to death.—Their words admit that they did not possess the power of life and death, while they imply that they had sentenced Jesus to death. They verbally give up the power, but in reality claim it, and regard the procurator as their executioner. The Jews had lost this power since the time that Archelaus was deposed, and Judæa became a Roman province (A.D. 6 or 7). The Talmud speaks of the loss of this power forty years or more before the destruction of Jerusalem. (Compare to Lightfoot’s Note here, and in Matthew 26:3.)

On the stoning of Stephen, which was an illegal act, compare to Notes on Acts 7:57 and following.