Joel Marcus, Mark 8-16: A New Translation with Introduction and Commentary (New Haven, CT: Yale University Press, 2009), 930.
While it is probable…that some members of the Jewish hierarchy pressed Pilate to put Jesus to death (Mark 15:1, 3, 11), and while a revolutionary ringleader may have been released at about the same time that Jesus was condemned, the Markan description of the Barabbas incident presupposes an implausible Roman custom for which no external evidence exists (15:6-8). This account may partly betray a theological interest, to depict the human perversity that valued a murderer’s life above that of God’s Son, and it may partly reflect the later Markan situation, in which some Christians have suffered from the violence of revolutionaries who, in their own day, have usurped the leadership of the Jewish community….
The Amateur Exegete 
There’s definitely a theological interest betrayed in that story, but I don’t think it’s about human perversity.
So, first, Barabbas is jailed because he killed someone in an insurrectionist uprising near the beginning of the Passover. There -is- external evidence for that event (the insurrection) from Josephus (War 1:88). There’s also a little of the criterion of embarrassment at play as some Matthew manuscripts list his name as “Jesus Barabbas,” but this seems to have been eliminated by scribes. Perhaps they felt it was improper that Barabbas should share Jesus’ name, and it may be why Pilate is depicted as highlighting “Jesus who is called the Christ” to distinguish him from Jesus Barabbas.
But this is all backdrop for the theological point. Barabbas (literally “son of the father”) is an insurrectionist compared to Jesus the Christ who has continuously warned that insurrection will bring about the destruction of Jerusalem. So, the crowd’s choice of Barabbas isn’t about their depravity in choosing a murderer over the Son of God – it’s about their choice of a path of insurrection over a path of repentance and a return to faithfulness to Israel’s God, which is the path of salvation that Jesus offers to them. Instead, they choose the broad way that leads to destruction.
All that to say that the story is more about politics than a generic statement about the crowd being bad people who like murderers better than Jesus. That, of course, doesn’t make it any more likely to have actually happened.
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Seriously, though. This story does not pass the smell test in any way at all. There is no way in this world or the next that Pontus Pilate lets an insurrectionist walk free after spilling Roman blood. Add that to the ridiculous “theory” that there was a Roman holiday where the procurator released prisoners after polling the crowd for the “winner” and you have another creative device for moving the blame for the crucifixion from the Romans to the Jews.
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