I am going to answer this with a long response I composed to answer a SIMILAR question..and what you got right is that this was a fabrication in the New Testament. I've researched this for decades. Here now is my answer:
Let us examine this a little more carefully.
Jewish history and Roman history of the supposed lifetime of Jesus is quite mute on the topic of his existence or any of the miracles claimed to have been performed before multitudes.
Nearly two generations pass before a replacement theology appears that demonizes Jews for not accepting a dead Jewish man as a form of god-man who can pardon sin, concepts antithetical to Torah.
Men to whom the Jews placed the hope of being the Davidic Messiah appeared in Jewish history both before and after the life of Jesus. As each one failed, they were never sought to be killed. A man hoped to one day rule Israel and break the yolk of religious and political persecution for the Jews first and then all humanity would of course not be a threat to the Jewish people. He would only be a threat to the oppressors or enemies of Israel/Jews.
Thousands of Jews were crucified by the Romans. Crucifixion was not a form of execution permitted in Torah.
The Christian religion's very concept of the term messiah is at complete odds with both the core tenet of faith in God presented in the Torah and the concept of the Davidic messiah prophecied for Israel.
They assign a function of pardoning sin and granting salvation to the job description of Messiah. That has ABSOLUTELY NOTHING to do with the prophecy of the Messiah in Tanakh.
Christianity takes many terms and words from Judaism but assigns foreign meanings to them. Some of the meanings are even directly forbidden by commandment in the Torah. THIS is the irreconcilable diffference in the two faiths, the nature of God and how humans connect to God and our fellow man! Judaism, through Torah has always taught that the righteous of all nations are blessed of God and also can know God directly.
Gentiles appear to forget or ignore that even the Temple had a court for the Gentiles. Isaiah refers to the righteous gentiles having a place in the world to come..EQUALLY!!!
So the Christian concept that they could only be saved through Jesus is foreign to Judaism and incompatible with what we are taught from the Tanakh.
Christians to say that Jews are awaiting any Christian concept of messiah are wrong because the Christian concept of messiah has nothing to do with Jewish prophecy
The New Testament wishes to demonize the people who adhere to the faith that it's concepts of faith are designed to replace.
Jews refused then to abandon the eternal covenant of Torah and Jews still refuse to do so. The New Testament claims in more than one passage the covenant of Israel was done away with. Daily, Christians tell Jews in Yahoo our faith was "nailed to the cross"..despite the continued existence of this covenant and faith 2,000 years after it's claimed to have been done away with.
The story of Barabbas was FABRICATED to demonize Jews. The circular reasoning of this story and some of the elements in it are hard to palate if one is at all familiar with Hebrew or Jewish history or Roman history for that matter. I assert it was fabricated since there has NEVER been any other mention of the non-existent "custom" of pardoning a prisoner for Passover anywhere else outside this story..designed to claim that the Jews could have 'saved' the "savior" ..and instead chose the criminal who had a literal name of the "son of the father" rather than the son of God. Hmmm..
Now..since the VERY first appearance of that story, Jews have asserted that it was ridiculous for several reasons.
1) No such custom associated with Passover existed..no reason for it to exist..it did not fit with any known Jewish law.
2) No Roman record of there ever being any pardons for Passover in the entire time of their occupation of Judea exist...and there ARE archaeologic records of much of their goings on in Judea when it came to edicts and proceedings.
3) The Romans were crucifying and persecuting Jews. If there HAD been such a bizarre custom...WHY would the persecutors honor that custom when they honored no other customs, and in fact sought to violate them to offend Jews to assert dominance.
They also place a catch 22 onto the Jew.
was it murder or willing self sacrifice?
was it a human or was it a deity who died?
It made no sense.
I could go on to list lengthy explanation to show how and why the story of the trial also makes similarly styled embellishments and fabrications about the Sanhedrin that are even more illogical in the face of historic record, but what's the point? Christians believe as "Gospel truth" the stories about Jews written in their holy scriptures no matter what evidence may exist to the contrary..
The New Testament really is the source of most antisemitism and haters of other stripes (fundamentalist Islamists through demonizing passages in Quran) simply took that and ran with it...with their own version of replacement theology.
This may sound harsh, but there is no other conclusion that can be made from an exhaustive study of history and the texts in question.
Simply read many of the answers here from Chrisitans and tell me if they do not take those demonizing passages serious and truthful?
Justify demonizing someone and it is not at all a "leap" of faith to justify turning your back or participating in persecution.
I think that the ONLY WAY that people can overcome these aspects of their dogma is to recognize the historic perspective and the reason those passages were put there..
AND I thank GOD that not all Christians follow what those passages lead to in practice
and that they recognize that it is our brotherhood as humans, and as children of one God to live more as an example of a walk with justice and mercy and compassion...the SERMON on the MOUNT and the Lord's Prayer are the core essence of what Jesus taught..and much has to be sifted out of the New Testament to gain that.
I simply cannot ignore what is said here by people who claim to hold the New Testament dear as infallible in every word and the history of what has happened and continues to happen because of the expression of such beliefs.
The New Testament story of Jesus' trial turns Jewish law and Jewish history recorded by Jewish scholars about the doings of the Sanhedrin during that time..topsy turvy.
The New Testament writers are TRUSTING to the ignorance of their readers about Jewish law and about the established procedures of the Sanhedrin that were taken EXTREMELY seriously , so that they don't get caught in those embellishments. For the most part..because most people are not interested in learning about the ancient Jewish system..they've been successful.
For instance, they do not know that a preliminary hearing of any kind, such as they allege to have taken place at the residence of Annas, or Caiphas( for even on that point there is no congruence in the Gospel accounts) could not possibly have been held. It violated their procedure and law they were swon to uphold. The whole court simply threw out their vows? Absurd.
They do not know that for a session of the Sanhedrin, it could NOT convene until the morning service at the Temple was completed. They also were apparently unaware that the Sanhedrin NEVER met on a Friday, nor on the eve of a Pesach ( Passover) Those were unalterable traditions..as well as the fact that for the charge of a crime that was capable of being sentenced to capital punishment, one had to have a mandatory appeals process and there could be FIVE such appeals before sentence was carried out. IN addition..NO sentence was to be passed on the same day as the trail was held.
The Sanhedrin was comprised of members of exceptional learning and character. No man could be a member of the Sanheddrin who had not previously filed three offices of "gradually increasing dignity" who was also not learned in sacred law, be free from "haughtiness" and never have been occupied in a trade or profession for the sake of finanacial gain or profit, he must be married and could not be a gambler, slave dealer or usurer, ..the list goes on to indicate that the members of the Sanhedrin were the most respected and given the task of upholding the Torah..to claim they behaved in such a manner as the New Testament depicted..is more absurd than to claim the United States Supreme Court( for which there exist none of the strictly enforced prerequisites of character) would hold a capital murder trial in secret and have witnesses act out in the manner of a kangaroo court with people screaming and acting out with no decorum whatsoever.
To the "objective" observer who knows these facts of Jewish history about the proceedings of the Sanhedrin..the story claimed about it does not ring true at all.The picture of a confused and wild scene at the crack of dawn at the High Priests residence where they throw all Torah law they are sworn to uphold and have spent their lives defending..sounds utterly ridiculous. Not only do they not follow any procedure of testimony of character witness for the defense first..but they allow witnesses to barge in and shout, they tie Jesus hands during interrogation and he is spat upon!
This is a long list of inconsistencies and I am leaving out many more just for the sheer length it would take to explain them all.
The High Preist is shown rending his clothes in a place and at a time when he is forbidden by Torah to do so! So it is obvious that the Sanhedrin trial of Jesus simply did not take place as it is depicted at all. Jesus did not appear before the Sanhedrin.
It was ONLY the Romans that a messiah hopeful or pretender would even threaten.
The Jewish idea of a Jewish republic with God as the master authoirity and the Torah as the law of the land would only threaten the Roman overlords. Pilate would not look upon the leader of a band of Galilean rebels as a harmless teller of religious tales..yet because the Romans even knew that the messiah must be an anointed KING..they mocked him as " king of the Jews"..
Pilate and the Romans were not PLACATING JEWISH WISHES..because what the Christian New Testament depicts as being done by the Jews flies in the face of any Jewish belief whatsoever. And from the VERY first appearance of those stories IN the New Testament..Jews have known that it was simply NOT TRUE that the Jews were the people who either sought or killed their savior deity ( sacrificed human for sin ) whichever it is they believe ( as I recognize not all Christians agree on whether he was man or deity) Pilate would have been remiss in his duty to Rome if he had not halted this rebel. And to make Pilate..a man whose own record reveals his cruel streak..the innocent victim of the wishes of the Jewish mob..also shows him to be a weak and ineffective leader of Roman rule and that doesn't fit Roman history, either.
So..now that you have a little more background into the Jewish history..do you see why it's not so objective to think that the Romans were appeasing the Jewish COURT?
Messiah hopefuls have lived both before and after Jesus and the Jews never sought to kill any of them.
In fact..look up about the life of Simeon Bar Kochba who lived after the time of Jesus. He actually was an anointed king of Israel for a time ( and left archaeologic record and evidence CONTEMPORARY to his lifetime)...but also failed at the job. Jews didn't want to kill him, either.
EDIT: TO the respondent who claims it was a Roman custom and they have record of this. Please produce a scholarly reference to any Roman record showing the pardon of prisioners for Passover.
And quite obviously, most Christians do understand the New Testament to declare that it was a Jewish custom as the passage specifically states. In fact, there is a record of a famous debate that included this particular aspect of the New Testament in its proceedings. By order of King James I of Aragon (Spain), Nachmanides (Rabbi Moses ben Nachman, 1194-1270) was compelled to participate in a public debate, held in the king's presence, against the Jewish convert to Christianity, Pablo Christiani. His brilliant defense of Judaism and refutations of Christianity's claims served as the basis of many such future disputations through the generations. The "Privilege Pasqualum" was one of the issues that Nachmanides dispelled.
to the person who said the texts do not say it is a Jewish custom, read your Bible again.
Just for ease of discovery I made a quick google and copied the last portion below from wiki..go look up the passages yourself please. I've read them.
Three gospels all state unequivocally that there was a custom at Passover during which the Roman governor would release a prisoner of the crowd's choice: Mark 15:6; Matt. 27:15; John 18:39. The corresponding verse in Luke (Luke 23:17) is not present in the earliest manuscripts and may be a later gloss to bring Luke into conformity. The gospels differ on whether the custom was a Roman one or a Jewish one.
Such a release or custom of such a release is not recorded in any other historical document.