Why did he do it? He was one of Jesus' chosen followers and friends.
John 6:70-71 tells us that Jesus knew Judas would eventually betray Him: "Jesus replied to them, 'Did I not choose the Twelve of you? Yet one of you is a devil.' He meant Judas son of Simon Iscariot, since this was the man, one of the Twelve, who was to betray him." John 12:5 also accuses Judas of being a thief.
Luke tells us that the bargain took place before the Last Supper (which, of course, we commemorate tomorrow):
"The feast of Unleavened Bread, called the Passover, was now drawing near, and the chief priests and the scribes were looking for some way of doing away with him, because they were afraid of the people. Then Satan entered into Judas, surnamed Iscariot, who was one of the Twelve. He approached the chief priests and the officers of the guard to discuss some way of handing Jesus over to them. They were delighted and agreed to give him money. He accepted and began to look for an opportunity to betray him to them without people knowing about it." (Luke 22:1-5)
Matthew fills in the details about the thirty pieces of silver:
"Then one of the Twelve, the man called Judas Iscariot, went to the chief priests and said, 'What are you prepared to give me if I hand him over to you?' They paid him thirty silver pieces, and from then onwards he began to look for an opportunity to betray him." (Matthew 26:14-16)
John, too, tells us that before the Last Supper, the devil had already put it in Judas' mind to betray Jesus ("They were at supper, and the devil had already put it into the mind of Judas Iscariot son of Simon, to betray him." John 13:2). We then see Jesus accepting this fate during the Last Supper:
"At that instant, after Judas had taken the bread, Satan entered him. Jesus then said, 'What you are going to do, do quickly.' None of the others at table understood why he said this. Since Judas had charge of the common fund, some of them thought Jesus was telling him, 'Buy what we need for the festival,' or telling him to give something to the poor. As soon as Judas had taken the piece of bread he went out. It was night." (John 13:27-30)Mark, however, has Jesus being perhaps a bit less accepting of Judas' role in what He knew was about to happen: "Yes, the Son of man is going to his fate, as the scriptures say he will, but alas for that man by whom the Son of man is betrayed! Better for that man if he had never been born.'" (Mark 14:21)
We are then told that in the Garden of Gethsemane, Judas betrayed Jesus with a kiss (Matthew 26:47, Mark 14:43-45, Luke 22:48).
Matthew gives us an account of Judas' death:
And finally, we see Matthias being chosen to take the place of Judas as one of the Twelve:
"When he found that Jesus had been condemned, then Judas, his betrayer, was filled with remorse and took the thirty silver pieces back to the chief priests and elders saying, 'I have sinned. I have betrayed innocent blood.' They replied, 'What is that to us? That is your concern.' And flinging down the silver pieces in the sanctuary he made off, and went and hanged himself.
The chief priests picked up the silver pieces and said, 'It is against the Law to put this into the treasury; it is blood-money.' So they discussed the matter and with it bought the potter's field as a graveyard for foreigners, and this is why the field is still called the Field of Blood. The word spoken through the prophet Jeremiah was then fulfilled: And they took the thirty silver pieces, the sum at which the precious One was priced by the children of Israel, and they gave them for the potter's field, just as the Lord directed me." (Matthew 27:3-10)
"One day Peter stood up to speak to the brothers-there were about a hundred and twenty people in the congregation, 'Brothers,' he said, 'the passage of scripture had to be fulfilled in which the Holy Spirit, speaking through David, foretells the fate of Judas, who acted as guide to the men who arrested Jesus- after being one of our number and sharing our ministry. As you know, he bought a plot of land with the money he was paid for his crime. He fell headlong and burst open, and all his entrails poured out. Everybody in Jerusalem heard about it and the plot came to be called "Bloody Acre", in their language Hakeldama. Now in the book of Psalms it says: Reduce his encampment to ruin and leave his tent unoccupied. And again: Let someone else take over his office. 'Out of the men who have been with us the whole time that the Lord Jesus was living with us, from the time when John was baptising until the day when he was taken up from us, one must be appointed to serve with us as a witness to his resurrection.' Having nominated two candidates, Joseph known as Barsabbas, whose surname was Justus, and Matthias, they prayed, 'Lord, you can read everyone's heart; show us therefore which of these two you have chosen to take over this ministry and apostolate, which Judas abandoned to go to his proper place.' They then drew lots for them, and as the lot fell to Matthias, he was listed as one of the twelve apostles." (Acts 1:15-26)