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.
Kiss of Judas
According to Matthew 26:50, Jesus responded by saying: "Friend, do what you are here to do". Elaine Pagels and Karen King have speculated that Jesus and Judas were actually in agreement with each other and that there was no real betrayal. Luke 22:48 quotes Jesus saying "Judas, are you betraying the Son of Humanity with a kiss?"