He was a thief, and helped himself to what was in the moneybag. How did John know? Matthew, a tax collector, might have spotted small discrepancies easily enough, and told Jesus, and John, close to Jesus, heard of it. Was Judas afraid of exposure, unwilling to repent?
I've read it proposed that Judas, knowing Jesus' power, wanted Him to quit dilly-dallying and use that power and popularity, and so tried to force Jesus' hand. It didn't work the way Judas hoped, hence his despairing not-quite-repentance. It's quite plausible, but not supported by the texts.
Or perhaps he was jaded with miracles. Many others saw the same miracles and merely got angrier with Jesus, and not at all interested in following Him. Jesus had said some things that would be really hard for a good Jew to listen to ("eat my flesh"), and maybe Judas was wondering if Jesus was really good. We're told that the disciples, when on mission, were also doing miracles--perhaps Judas did some miraculous healings too. People are really good at forgetting inconvenient things, but that would be a doozy to try to forget.
Or perhaps he was sloppy and the devil slowly took hold of him.
Or perhaps his motives were a mix of all of the above.
I heard it asked if Judas had a choice; if the prophecy meant that somebody had to betray Christ. I think that's a bit backwards. If Judas had chosen otherwise, the prophets would have been told something else to say.
It's been claimed that Peter is Everyman, standing in for us all, denying Christ through fear and surprise. But so is Judas, betraying Christ and perhaps not entirely sure why. And so is Thomas, doubting the testimony. And so is John, loved by Christ.
