Barabbas was most likely a leader of a group of Jewish revolutionaries called Zealots. The Zealots fought against the Roman occupation of Israel. Extremists among the Zealots turned to terrorism and assassination. They became known as Sicarii (“dagger men”). They frequented public places with hidden daggers to strike down persons friendly to Rome - even other Jews.
The Zealots used the Biblical prophecies of the Messiah crushing the enemies of Israel as a basis for their rebellion. The Gospels make reference to Barabbas as a 'notorious prisoner,' 'insurrectionist,' and murderer. It was most likely for terrorism and assassination that Barabbas was in prison.
Some who followed Jesus thought He might be the chosen Messiah to overthrown the Roman occupation, but later became disillusioned when Jesus spoke of turn the other cheek, and love your enemies, rather that take up arms and slaughter them.
From Pastor Bob Kaylor: "In short, Jesus of Nazareth, Son of God, didn’t fulfill the visions of the zealots. His revolutionary-minded disciples were no doubt confused by this all the way through their time with him — a revolution with no swords? Some of his hangers on began to leave him at this point. In fact, some scholars suggest that one of the reasons Judas betrayed Jesus was out of his disappointment that the revolution hadn’t begun. Maybe by getting him arrested it would cause an uprising. But it didn’t and Judas was left alone holding the bag.
If Barabbas had ever encountered Jesus before that day in the Praetorium, he might have seen him as weak and ineffective, just another holy man without a real plan. And yet there is Jesus on trial, accused of doing the very thing of which Barabbas himself was absolutely, unapologetically guilty. Jesus of Nazareth, Son of God, had done nothing under Roman law to deserve a cross, while Jesus Barabbas deserved nothing less."
Das Wiki explains how these events transpired: "According to all four canonical gospels there was a prevailing Passover custom in Jerusalem that allowed or required Pilate, the praefectus or governor of Judea, to commute one prisoner's death sentence by popular acclaim, and the "crowd" (ochlos), "the Jews" and "the multitude" in some sources, were offered a choice of whether to have either Barabbas or Jesus released from Roman custody.
According to the Synoptic Gospels of Matthew, Mark, and Luke, and the account in John, the crowd chose Barabbas to be released and Jesus of Nazareth to be crucified. Pilate is portrayed as reluctantly yielding to the insistence of the crowd."
"Then Pilate, when he had called together the chief priests, the rulers, and the people, said to them, “You have brought this Man to me, as one who misleads the people. And indeed, having examined Him in your presence, I have found no fault in this Man concerning those things of which you accuse Him; no, neither did Herod, for I sent you back to him; and indeed nothing deserving of death has been done by Him. I will therefore chastise Him and release Him” (for it was necessary for him to release one to them at the feast).
And they all cried out at once, saying, “Away with this Man, and release to us Barabbas”— who had been thrown into prison for a certain rebellion made in the city, and for murder.
Pilate, therefore, wishing to release Jesus, again called out to them. But they shouted, saying, “Crucify Him, crucify Him!”
Then he said to them the third time, “Why, what evil has He done? I have found no reason for death in Him. I will therefore chastise Him and let Him go.”
But they were insistent, demanding with loud voices that He be crucified. And the voices of these men and of the chief priests prevailed. So Pilate gave sentence that it should be as they requested. And he released [g]to them the one they requested, who for rebellion and murder had been thrown into prison; but he delivered Jesus to their will." Luke 23:13-25
This image is in the public domain. It's from volume 9 of 'The Bible and its Story Taught by One Thousand Picture Lessons,' edited by Charles F. Horne and Julius A. Bewer, published in 1910.