The Jewish leaders knew Jesus was the Messiah to come. We are told this in the parable Jesus gave in Matthew 21:33-40. They knew He was the Messiah but their authority was challenged. This is why they killed all the prophets before as well.
While the Jewish leaders knew Jesus was the Messiah, it is apparent that the Jewish people still had questions. They were expecting a leader to come and free them from the control of Rome and re-establish Israel (Acts 1:6). This misbelief is likely what fueled the crowds to yell, "crucify Him!"
Before we talk about Barabbas we must get a preconceived notion out of our mind. While Barabbas is portrayed by Hollywood as an ugly and mean looking person, we don't really know what Barabbas looked like.
Many people understand Barabbas to be a murderer and that's about all that is taught about him. The truth is, Mark and Luke tell us he caused insurrections against Rome (Mark 15:7; Luke 23:19). This action by Barabbas would be closer to what the crowd was looking for in the coming Messiah.
Another thing that is ignored by modern Christians, especially those in America, is the meaning of names. We don't regard names beyond the point that it's what an ancestor was named, what we like, etc. In ancient days, names had meanings and people looked seriously at those meanings. The name Barabbas literally meant "son of the father".
Now we can see how so many people, desperate to be freed from the clutches of Rome, could be deceived into selecting the wrong person. They had a choice between a man who seemed to have the power of God but who submitted to Rome authority rather than conquering them. And then there was a man whose name meant "son of the father" who was an insurrectionist against Rome.
We must understand that we can be deceived so easily based upon what we think is supposed to be and we can make all the wrong decisions. This is why we need to study Scripture without our preconceived notions and see what is really being said rather than following along with the crowd.