Why did the crowd choose to free Barabbas the murderer but wanted Jesus crucified and what does this tell us about human behaviour?

In the crucifixion story, the Jews were asked to choose between Jesus, the Son of the Father, and Barabbas, whose name also means, in Hebrew, Son of the Father. They chose to release Barabbas and to crucify Jesus.

John Shelby Spong (Jesus for the NonReligious) says that in his research, Spong has been able to find no evidence that there was a custom of releasing a prisoner at the time of the Passover. He says the original crucifixion story could have been related to Yom Kippur. One lamb or goat was killed for our sins and one had the sins of the people symbolically transferred to him, after which he was chased away. Mark simply transferred the imagery from the Yom Kippur festival and applied it to the trial of Jesus. in the Passover season.

On Shelby's reading, supported by others such as MacDonald, Jesus and Barabbas represented the two sheep of the Jewish tradition, one of which must be released and the other must be sacrificed. By releasing Barabbas, the Jews chose the wrong 'scapegoat'. Of course this tells us more about the development of the gospel than it does about human behaviour.