First, some clarification. Crucifixion was a Roman form of execution, so only Roman officials had the legal authority to call for it. That means the story of Jesus's death on the cross requires that a highly placed Roman play some role.
Enter Pontius Pilate.
Just like Herod didn't want to execute John at first, Pilate is also hesitant to crucify Jesus. After all, Pilate knows that Jesus is innocent and that the whole charge is coming out of jealousy. But Pilate needs to keep order, and so he gives in to the crowd's demand that he crucify Jesus (15:2-15). Hmmm, that was easy.
The whole function of Pilate's characterization in Mark is to pin the blame for Jesus's death on the Jewish leaders, who brought Jesus to Pilate and incited the crowds to clamor for his crucifixion. Want to know what their guilt means in terms of Mark's broader vision? Check out "The Parable of the Wicked Tenants" and the theme of "Revenge."