Despite his ridiculously caricatured name, Pontius Pilate was a real guy. He was the prefect of Judea from 26 to 36 CE, and there's no doubt Luke knows it (3:1). His political jurisdiction included the city of Jerusalem, which for Luke means he oversaw Jesus's crucifixion.
While Luke definitely knows that Pilate was in charge, he shifts the responsibility for Jesus's death to the Jewish leaders and people, who demand Jesus's crucifixion (23:1-5, 13-25).
You might think this makes him a Nice Guy. And sure, at best, Pilate is a pushover. But at worst, he's a Terrible Person. Luke holds him responsible for slaughtering some Jews from Galilee while sacrificing in the temple at Jerusalem , and he's totally on board with Herod's mockery of Jesus (23:11-23).
And don't forget it. Understanding Luke's negative characterization of Pontius Pilate forces us into a pretty startling theological insight: even the most brutish of politicians can be agents of God's will in the human world. After all, Luke makes it clear that the crucifixion is the destiny woven by God for Jesus (9:22, 44; 17:25; 18:32-33), and Pilate is the one who makes the final call. Go ahead and chew on that for a while.