What does John 19:15 mean?

Though he knew Jesus was innocent (Matthew 27:18; John 18:36–38), Pilate had Him brutally whipped and humiliated, seeking to satisfy the mob's blood lust (John 19:1–4). That did not work; the crowd demanded Jesus suffer the most agonizing, shameful death possible (John 19:5–7). When the crowd feigned loyalty to Rome (John 19:12) and threatened to riot (Matthew 27:24), Pilate brought Jesus to the place of judgment. There, he also put on a show, as if he were in control of the situation (John 19:11). This posturing included acting as if he was in possession of Israel's "King," a title which this crowd had not applied to Jesus.

Here, the crowd repeats their vicious demands, and Pilate duplicates the mocking claim that he's passing judgment on the King of Israel. Coming from the governor, it's a way to deflect from the fact that he's been outmaneuvered. He will attempt to reinterpret this result as a strong Roman governor, cracking down on a rebellious Jewish people, by crucifying their purported king.

Jesus' enemies, for their part, cynically claim loyalty to the Roman Empire. The irony of this moment is heartbreaking: God sent His own Son to His chosen people. Those people are now demanding an earthly power murder this Messiah, as they proclaim their submission to men, rather than to God. The descendants of Abraham are shouting their praise for a pagan, ungodly empire and encouraging the death of their rightful King.