Good grief.
Barabbas was a murderer. He wasn't working with Jesus.
The gospel writers go to every conceivable length to absolve the Romans in general, and Pilate in particular, of Jesus' crucifixion and to blame it on the Jews. The reason, of course, was that Christianity was going to have to exist under Roman rule for many years, which is why the New Testament contains nothing critical of the Romans, even though they were hated for their heavy taxation, and Pilate was hated for his brutality.
For the church, the Jews made an appropriate scapegoat because the Jews were a thorn in side of the early church. The Jews, of course, had far greater knowledge of Jewish laws and traditions than the largely gentile church, and were able to call attention to some of the errors being taught by the church.
The Biblical account of Pilate's offer to release Jesus but the Jews demanding the release of Barabbas is pure fiction, containing both contradictions and historical inaccuracies.
What had Barabbas done?
Mark 15:7 and Luke 23:19 say that Barabbas was guilty of insurrection and murder.
Mark 15:7 And there was one named Barabbas, who was chained with his fellow rebels; they had committed murder in the rebellion.
Luke 23:19 who had been thrown into prison for a certain rebellion made in the city, and for murder.
But, John 18:40 says that Barabbas was a robber.
Then they all cried again, saying, “Not this Man, but Barabbas!” Now Barabbas was a robber.
Which one is it? John clearly contradicts what is written in Luke and Mark
Pilate's "custom" of releasing a prisoner at Passover
This is pure invention, the only authority given by Rome to a Roman governor in situations like this was postponement of execution until after the religious festival. Release was out of the question. It is included in the gospels for the sole purpose of further removing blame for Jesus' death from Pilate and placing it on the Jews.
Pilate gives in to the mob
The gospels have Pilate giving in to an unruly mob. This is ridiculous in light of Pilate's previous and subsequent history.
Josephus tells us that Pilate's method of crowd control was to send his soldiers into the mob and beat them (often killing them) into submission. Pilate was eventually recalled to Rome because of his brutality.