What does Luke 23:15 mean?

Pilate is conducting a proper seven-stage Roman examination to determine if Jesus is guilty of a crime. He acts as if the Jewish leaders who charged Jesus with breaking Roman law are reasonable and logical men. He seems to assume they will accept this reasonable and logical ruling.

The Jewish leaders have accused Jesus of stirring up the people, telling them not to pay taxes, and declaring Himself king. Pilate doesn't think Jesus is a threat, but the Jewish leaders insist. To gain some perspective, Pilate sent Jesus to Herod Antipas, the ruler of Jesus' home territory Galilee. This resulted in a lengthy examination during which the chief priests and scribes passionately insisted Jesus was guilty. Jesus refused to speak. So, Antipas determined Jesus was innocent, as well (Luke 23:1–14).

Even earlier, the chief priests, elders, and scribes had held an illegal trial to try to pin down something with which to charge Jesus. They called numerous witnesses against Him, but none of their testimonies agreed (Mark 14:53–65). Here, they get their two witnesses: both Pilate and Herod Antipas declare Jesus to be innocent. Antipas's decision is especially poignant since the Pharisees either thought he wanted Jesus dead or lied about it (Luke 13:31).

Pilate knows they're agitated, so he offers a compromise: he will torture Jesus for a while and then release Him. Perhaps that will convince Jesus to stop making the Jewish leaders so excited (Luke 23:16). But the Jewish leaders—and the crowd they've agitated themselves (Matthew 27:20)—reject Pilate's offer. They want to see Jesus hang (Luke 23:18–21).