Mark 15:10

For he perceived that for envy the chief priests had delivered him up.

One thing to keep in view throughout is that Pilate was never for a moment deceived into believing that Jesus was a seditionist. He knew the essential facts of the whole dramatic event thoroughly, being insufficient only in his failure to see Jesus as God in the flesh. The inscription he affixed to the cross did not deny Pilate's knowledge of Jesus' innocence, but it was a sadistic joke on the Sanhedrin, from his point of view.