Some Political Background Info to the Crucifixion
The Romans were in political control of Jerusalem at the time of Jesus, and unsurprisingly there were Jewish rebels who tried to overthrow Roman rule. (The ancient Jewish historian Flavius Josephus–who lived a generation after Jesus– tells us a lot about these movements.)
I think it’s a fair assumption that Barabbas was a rebel leader, which is why he was imprisoned under Pontius Pilate, awaiting sentencing. Luke also tells us that Barabbas was a murderer. Most likely, he murdered Romans.
Think about the irony: Jesus, the innocent man, dies on behalf of Israel for his enemies, whereas Barabbas, the guilty man, goes free.