Judas Iscariot
There are several explanations as to why Judas betrayed Jesus. In the earliest account, in the Gospel of Mark, when he goes to the chief priests to betray Jesus, he is offered money as a reward, but it is not clear that money is his motivation. In the Gospel of Matthew account, on the other hand, he asks what they will pay him for handing Jesus over. In the Gospel of Luke and the Gospel of John, the devil enters into Judas, causing him to offer to betray Jesus. The Gospel of John account has Judas complaining that money has been spent on expensive perfumes to anoint Jesus which could have been spent on the poor, but adds that he was the keeper of the apostles' purse and used to steal from it.
Giovanni Canavesio
This classical motif chosen by Canavesio was when Christ announces his coming betrayal and indirectly hints on Judas as the traitor. Both Christ and Judas reach their arms toward the tray in the center while Christ holds a piece of food. The traitor is revealed as well in the illustrations of Matthew (26:23)—Judas dipping his hand in the dish—and John (13:26)—Judas receiving the soup. The interaction of Christ and Judas is signified by Christ extending his right hand and Judas his left. That Peter holding a knife can frequently be interpreted as his combative mood that is often presented in theatrical episode, Canavesio has no intention of showing this motif but is influenced by Savoyard Passion Cycle.