This is actually an interesting part of the new testament. Judea was controlled by the Romans at the time of Jesus' life, and the people living there didn't really like that. There were attempts at rebellion all the time. At passover it was customary for one person to be pardoned for their crimes, chosen by the locals. Jesus was this guy who started claiming to be the King of the Jews, and at the same time tried to reform the church. This was, in all ways, blasphemy and illegal, so he was arrested when he entered Jerusalem. But there was someone else who could have been chosen for the pardon, A man named Barabbas. He was part of a group that attempted a violent rebellion against the Romans. He was a straight-up revolutionary to the people there. So when it came to a guy who fought for their independence or a guy who called himself king and kept promising eternal salvation if you were peaceful and nice to sinners, well, it's pretty obvious what choice the crowd would choose.