Pilate is one peculiar character in the Bible. On one hand, Pilate saw that Jesus was innocent -- and Pilate himself said that he "[found] no fault in Him" (19:4). On the other hand, however, Pilate "scourged [Jesus]" with his soldiers "twist[ing] a crown of thorns [...] on His head" while mocking and striking Jesus (19:1-3).
Pilate likely gave in to public pressure. Although he knew the innocence of Jesus, the Judean crowd basically cried out for prosecution of Jesus. This isn't the last time that Pilate gave in to public pressure regarding Jesus.