Luke 22:3

And Satan, entered into Judas who was called Iscariot, being of the number of the twelve. And he went away and communed with the chief priests and captains, how he might deliver him unto them. And they were glad, and covenanted to give him money. And he consented, and sought opportunity to deliver him unto them in the absence of the multitude.

What probably triggered Judas' treachery was the rebuke administered to him by the Lord during the incident of the anointing in the house of Simon the leper (Matthew 26:6ff).

With Judas on their side, as they supposed, the chief priests then thought that they would procure ample evidence to warrant a public trial and judicial execution. As it turned out, Judas returned the money in bitterness and remorse, refusing to have any further part with the religious leaders; but it was too late for them, as well as for Judas. The whole shameful episode would be spread upon the public record of the ages.