Judas was the one that would betray Jesus. Think about how many times Jesus could have been arrested teaching or walking in a public place. It would not be one of the Pharisees or rulers who delivered Jesus up. It would be a traitor from His own twelve disciples. This is the source of the emotion in Jesus from verse 21. He is betrayed by a friend. How many of us have been betrayed by someone close: a schoolmate, a family member, a close friend who turns on you or fails you. Jesus knows your hurt and He can identify with you because He too was betrayed by a friend.
You can expect to be let down by people. They will either fail you because of their weakness or sinful selfishness. There is one friend who will never fail and it is Jesus Christ.
Consider: Romans 8:31-39