This morning I was reading in John and it mentioned that Judas Iscariot was known to have dipped his hand in the til from time to time. The disciples are considered to be some of the greatest followers and yet Judas was a known crooked person and walked among them. There's a chance that since John wrote his book well after the Resurrection that he was "editorializing" and adding facts they only figured out afterwords. I don't imagine after Easter Sunday they sat around auditing the late-Judas' books to figure it out. One of them had to know beforehand. It is te same where you work, there's gossip to be heard--or shared. Judas was a Judas before Judas meant Judas and the disciples knew it. More important, Jesus knew it.
But he let him stay. He left him in charge of the money. Clearly the money tempted Judas, but Jesus let the temptation ride. He did not rescue Judas from himself. The man who raised the dead, healed the sick, turned water into wine, and spoke in parables the people understood even when his followers didn't did not save one of his 12 closest and personally chosen friends from himself. C.S. Lewis said in Mere Christianity, "If God thinks this state of war in the universe a price worth paying for free will...then we may take it it is worth paying." Jesus let Judas be Judas because it was a price worth paying for all of us.