strange ending

IMHO, Barabbas at the end was giving himself up to his insecurity (darkness) about his belief in Jesus, and was hoping that Jesus would recognize him and his efforts to believe in Him, and forgive him for being the one whose place he took on the cross.

In Mark, 9:14-29,
From the story of the possessed boy who is brought to Jesus by his father to have the evil spirit cast out is heard from the boy's father when Jesus says to him: "All things are possible to him who believes." In agony the boy's father calls out, "Lord, I believe. Help my unbelief!" Jesus, recognizing the man's struggle, commands the evil spirit to leave the boy. Doubt is a part of the faith journey, but if we sincerely try, there is the hope of heaven. I believe that this was the case in the end with Barabbas. He still had the hope that Jesus would recognize his struggles and his name and receive him. Tradition says that Barabbas' name was Jesus bar Abbas.
I think that many of us who struggle pray with the possessed boy's father and with Barabbas, "Lord, I believe, help my Unbelief!"

Alternatively, Barabbas misunderstood as others did, among them, it seems, Judas, who Jesus was, those who expected Him to come as a warrior Messiah. That is symbolized by Barabbas' misguided attempt to help what he thought was Jesus' purpose by setting fire to Rome.
His despair at the end was because he finally realized he had never understood Jesus. His understanding was so limited by his brutish way of life that the spiritual realm over which Jesus presided was beyond his grasp.