- frustrated with a lack of political ambitions: Judas was looking for the Messiah to be the new king. This opinion comes from an interpretation of the Greek version of Judas' name and its similarity with sicariis, a name for the band of assassins that targeted Romans and those who sympathized with them. However, the high priests were appointees of the Romans, so why would a terrorist conspire with one of his targets? (Probably Iscariot comes from the Hebrew ish Kerioth--man of Kerioth, a border town near Edom, Herod the Great's homeland--a long way from Galilee.)
- desire for more money: there is a bit of evidence that perhaps Judas was embezzling from the disciples' funds. Did he take advantage of the religious leaders' dislike of Y'shua as an additional source of income?
- purely Satanic influence: it is what Luke mentions as the proximal cause of the betrayal. What temptation would Judas have surrendered to in order for Satan to gain control? Are other followers of Christ subject to Satan's influence? What is the chink in the armor that lets the Evil One in?
- self-preservation: detecting the unrest from the those in power, Judas may have seen the end coming. Y'shua himself predicted such. If He dies, then His followers might be rounded up, imprisoned and/or executed. In fact, this seems to be Simon Peter's motive for denying knowing Y'shua on the night of His trial. After His crucifixion, the disciples would gather behind locked doors, fearful of being arrested. (In fact, some were arrested and most later released.) This might be the strongest candidate for Judas' motive.
Likewise, a debate broke out over the Pharisees' and teachers' motives for plotting to kill Y'shua. Most thought it was because the Rabbi rebuked their adherence to 'oral' law and challenged their interpretations of the Torah. However, this was a common practice--to have question and debate over the meaning of scriptures. If they felt humiliated, this is an extreme reaction. What form of humiliation would drive you to conspire with a group to murder a man?
Capital punishment was prescribed in certain situations within the Torah: murder, rape, kidnapping, adultery as well as blasphemy, witchcraft, false prophecy, continued rebelliousness, cursing parents. Most likely, the Pharisees and teachers found some evidence of blasphemy or false prophecy, or kept watch to see if Y'shua might commit those sins. There is some Rabbinic tradition, however, that suggests if the Sanhedrin sentences one person to death in a seven-year period, that that is too many times. Others had said once in 70 years was too many times to invoke capital punishment. In Christ's time, this group is bucking tradition if they are so upset or zealous for the law that they want to arrest Him and condemn Him. (Later, they do arrest Stephen and condemn him to stoning. Other disciples are just imprisoned for a brief time.)
We may never know. Chances are we'll project our own motive for similar behavior onto Judas and the Pharisees. If scripture is silent, it may not be important. Only the act was. Or maybe not since God does care about our hearts. In any case, it was all part of God's plan to save us all. He needed some players just like He needed Pharoah to resist many times in order for God to reveal His might to His people and the Egyptians, and the rest of the world at that time.
His will won't be thwarted. He'll find the right people with the wrong reasons in order to the right thing to show His glory.
