Pontius Pilate was well aware that he was being manipulated by The Sanhedrin's representatives, determined to persuade the Roman administrator that Jesus posed a threat to the cohesiveness of Israeli society.
Pontius Pilate was a civil servant not drawn from a military background attempting to maintain peace in a troubled Roman province. While recognising that Jesus was innocent of the charges levelled against Him, the Roman administrator had to walk a very fine tight rope ensuring that he kept the support of The Sanhedrin.
That the Jerusalem mob chose Barabbas, over Jesus ensured that Pontius Pilate could wash his hands of the matter. The rest of the story is history:
Probably old news to most but new to me on reading Benedict's Jesus of Nazareth today, that Barabbas means 'son of the father'. An agent of insurrection against the Roman occupation. Some translations apparently name him Jesus Barabbas.