Only two people knew.  Jesus knew that Judas would betray Him, and Judas knew the contract he had entered: 30 pieces of silver exchanged for the body of Jesus (Matthew 26:14-16).  But Jesus was not the only one who had been betrayed.  The other disciples, although they knew that Judas was a thief (John 12:6), had no idea of the evil he had agreed to do (Luke 22:1-6).  So, when Jesus announced that one of them would betray Him, they were very sad. Matthew records, "And while they were eating, he said, 'TruIy I tell you, one of you will betray me.'  They were very sad and began to say to Him one after the other, 'Surely not I, Lord?'" (Matthew 26:21-22).
 
John's comment in verse 30 sums it up: “It was night”.  Not just physically dark in the late evening, but morally black and spiritually wicked.  For over two and a half years, one dark heart had been so close to the Light of the World but had seen nothing.  No wonder Jesus was troubled in spirit.  Judas was so near to Him but so far away at the same time.  The greatest opportunity of all eternity had been missed.  He went out of the group because he could not stay in the light any more; and he never returned.  That same pattern has been followed by false believers in every age (1 John 2:19).  Every break in spiritual fellowship is painful, and Jesus felt it (Psalm 55:20-21).

At the time, the disciples did not realise what was happening, even though they asked.  But looking back, John fully understood the significance of Jesus’ words, the bread dipped in bitter savoury herbs and Judas’ departure.  Worse still, John realised that Judas had allowed himself to be used by Satan, who entered into him.  This was not just human greed to get thirty pieces of silver, but the devil’s cosmic hatred which was intent on destroying Jesus.  Indeed, no night could be darker.
 
The church is a wonderful group of people.  But it is possible to be in a church, even sing or serve, even preach or pray, but still be in the dark.  Although the Judas story is unique, the pattern is common.  Often, like the disciples, we do not know the identity of the dark heart.  But Jesus does.  When it becomes obvious, we all feel betrayed and sad, but none is sadder than Jesus.  And yet He longs to forgive repentant people and give them a second chance to receive His love.  Peter came back to Jesus and was repentant (John 21:15-22): Judas was only remorseful and lost everything.