What does Mark 15:27 mean?

"Robber" is from the Greek root word lestes. It refers to a plunderer or brigand. It is the same word John uses to describe Barabbas (John 18:40), although Barabbas is also described as an insurrectionist and murderer (Mark 15:7). Typically, the punishment for theft is repaying the victim many times the value of the stolen item. Crucifixion is only used if the theft occurs in a religious or royal building or if the thief kills someone in the execution of the theft. Scholars posit that these men are insurrectionists, possibly even Barabbas' accomplices, though we have no hard evidence of this.

The scene is reminiscent of an earlier event (Mark 10:35–45). On the road to Jerusalem, before Jesus and the Twelve had reached Jericho, James and John (and their mother; Matthew 20:20) asked Jesus if the two brothers could sit at Jesus' right and left when He established His kingdom. The disciples still thought this trip to Jerusalem might be the beginning of Jesus' revolt against the Romans and the early steps to free the Jews. Thomas, for his part, thought it was just as likely that they'd all die (John 11:16).

Jesus asked the two disciples if they could share His fate and then admitted they would. He didn't mean glory and authority, however. He meant rejection by the Jews and death in defense of God's kingdom. Jesus then explained, again, that leadership in His kingdom means humbling oneself and serving others, even to the point of death. Jesus also told James and John that He did not have the authority to say who would be at His left and right.

Here on the cross, Jesus embodies the full expression of sacrificial service and the manifestation of the kingdom of God. James and John should be grateful two robbers have taken the places they requested: at the side of Jesus. The seats to the left and right of the ruler are typically for advisors. James and John can no more advise Jesus than these robbers. One of the robbers realizes this (Luke 23:39–43). He confesses he is powerless and only Jesus can save him. He has no right to advise or judge Jesus. In response, Jesus doesn't offer him a position of authority, He promises him salvation.

James and John do eventually share aspects of Jesus' fate. James is the first of the Twelve to be martyred, beheaded by command of Herod Agrippa (Acts 12:1–2). John will suffer greatly before he dies of old age. Their mother is watching Jesus die. She is Salome who stands at a distance with Jesus' mother and Mary Magdalene (Mark 15:40). For now, they are all a safe distance away.