What does Mark 14:21 mean?
Jesus often calls Himself the "Son of Man," a phrase taken from Daniel 7. Daniel has a vision in which the "Ancient of Days" (God the Father), gives "dominion and glory and a kingdom, that all peoples, nations, and languages should serve him" to "one like a son of man" (Daniel 7:14, 13). The disciples have readily accepted Jesus' identification as the Son of Man. It means that He is the Messiah. They think this means He will at this time free Israel from foreign rule and bring in an age of peace and prosperity. And Jesus has promised the disciples will be on hand to take leadership positions in His court (Matthew 19:28).Jesus has also explained to the disciples that the Son of Man will suffer and die (Mark 8:31; 9:9, 12, 31; 10:33). They don't realize they are witnessing the very beginning of this movement. They think Jesus has sent Judas to get supplies for the Feast of Unleavened Bread to be held the next night (John 13:28–29). They don't know Judas has gone to ignite the darkest hours of human history.
Daniel, in the Old Testament, doesn't mention that the Son of Man will be betrayed, tortured, and killed. That fate is explained by the "Suffering Servant" imagery of Isaiah 53. Jesus has spent His time with the disciples explaining that the two figures are one unified person. First, the Suffering Servant will be "despised and rejected by men, a man of sorrows and acquainted with grief; and as one from whom men hide their faces he was despised, and we esteemed him not" (Isaiah 53:3). It is at His second coming that the Son of Man will be given His kingdom (Mark 13:24–27).
Ultimately, Jesus' comment about Judas is true of every unbeliever. It would be better not to have been born than to face eternal judgment in hell. Much ink has been spilled debating why, how, or even "if" God made us as eternal souls with the free will to choose or reject Him. Practically, Christians should use this knowledge and follow Jesus' example: preach the gospel and mourn those who reject it.