The thing that makes Judas so interesting to me is that the gospels give absolutely no motive for his actions. We're left to fill in the blanks for ourselves.
We can do that, as some have, by simply saying "He betrayed Jesus because he was evil," but falling back on a Saturday morning morality tale on good and evil does nothing to further our understanding of why a man would betray someone he has known and admired for at least three years.
The gospel of John says that Satan had entered Judas' heart, but even that's an overly simplistic understanding of things. Ha-Satan isn't even necessarily an evil being in the Bible, as much as he is an accuser or prosecuting attorney fulfilling an essential function in God's court.
I rather fear that Judas had what he believed were good reasons for doing what he had done, as we usually do when we do something wrong. Afterward, when he saw what had happened to Jesus, he was struck with remorse, saw his action for the evil it was, and hanged himself.
Motives I've thought of for betraying Jesus:
1) He was trying to force Jesus to act by making it so he had no choice but to rally the people and overthrow the Romans.
2) He had decided that Jesus was not the messiah after all, and so he betrayed him rather than see the Jewish people led astray.
3) He was afraid that Jesus was going to be a militaristic messiah, and realized the Romans would destroy the nation if he led an uprising. Similar to Caiaphas' prophecy, "Do you not know that it is better for one man to die than for a whole nation to perish?"
4) I wrote a drama once where Judas saw that everything had slipped out of control, and that Jesus was no longer atop the wave but about to be swept under it. Rather than see him go the same way as a dozen other messiahs, he chose to betray him so he would be martyred and people would remember him at his height, and hold up his teachings for an age to come.
5) A pettier reason might be found in the gospel of John: Judas was a thief. It's possible that he feared that Jesus was going to kick him off the team -- Jesus did identify him as the betrayer during the seder meal -- and so he saw this as a chance to spare himself the blow to his pride, and get 30 talents of silver, to boot.
I do think the "He did it because people are evil" sentiment is a grievous mistake in hermeneutics, however common a reason it may be. It robs us of any chance to learn from the story, because in identifying Judas as evil, we simultaneously disavow any connection between ourselves and him, as no one outside superhero cartoons ever views herself as evil. We all view our crimes, misdemeanors and sins as perfectly justifiable, because of the exigencies of the situation. Falling back on theology to defend such a position is to misappropriate doctrine for evaluating interpersonal relationships.