Pilate knows Jesus is innocent, and all things being equal would prefer not to execute an innocent man. So he tries to come up with an excuse for letting Him off, without having simply to decide for Himself that He should go free.
Mark 15:6 Now at the feast he used to release for them any one prisoner whom they requested. 7 And the man named Barabbas had been imprisoned with the insurrectionists who had committed murder in the insurrection. 8 And the multitude went up and began asking him to do as he had been accustomed to do for them. 9 And Pilate answered them, saying, “Do you want me to release for you the King of the Jews?” 10 For he was aware that the chief priests had delivered Him up because of envy. 11 But the chief priests stirred up the multitude to ask him to release Barabbas for them instead. 12 And answering again, Pilate was saying to them, “Then what shall I do with Him whom you call the King of the Jews?” 13 And they shouted back, “Crucify Him!” 14 But Pilate was saying to them, “Why, what evil has He done?” But they shouted all the more, “Crucify Him!” 15 And wishing to satisfy the multitude, Pilate released Barabbas for them, and after having Jesus scourged, he delivered Him to be crucified.
Note here the irony of the release of Barabbas: Jesus is falsely accused of leading a rebellion against Rome; Barabbas did lead a rebellion against Rome. The guilty one goes free; the innocent man dies. Pilate’s preference for not executing an innocent man is just that – a preference, not a conviction. So when the Jewish authorities convince Pilate not that Jesus is guilty but that it is in his personal interest to execute Jesus, he agrees to do so.
Who is Jesus for Pilate?
An innocent madman. Pilate thinks, “This fellow? The King of the Jews? What idiocy! He’s not in his right mind. Clearly he’s innocent and should be released, but, hey, to do so might cause a riot – and I can’t afford any more riots. I’d be putting my entire career at risk! My position is much more important than any Jewish madman. So let him die. It’s a pity – but he must die.”