Only Judas and God can really answer but we can guess. First remember that all fall short and sin. Judas' sin just happens to be notroious. But if you look at all the disciples- every single one of them doubted, disobeyed, and feared. They denied him, demanded to see his hands to prove himself, sank into the water after enthusiastically jumping out of a boat...
As Yabeya pointed out- Judas was a Zealot. Many Jews at the time were expecting a warrior king -not someone teaching peace and dieing for spiritual salvation. It was physical freedom they were expecting. Judas was most likely one of them. At some point everyone questions God because he is not what they think he SHOULD be.Why should Judas be any different? He wanted a physical salvation and may even feared that Jesus was guiding them away from that salvation.
The Jews saw God's powers directly. They witnessed plagues that they were saved from, witnessed a pillar of fire between them and a blood thirsty army, crossed the red sea on dry land with walls of water on each side, and yet they worshiped a gold COW because they could not be patient. Like all of us- Judas was human. He behaved in a very human fashion. He had dreams, hopes and confiction. Jesus was bringing a message he did not expect to hear.
Then again it could have been pure self preservation. Sacrifice the virgin to the valcano to save the village. Hand the Jews over to Hitler so that you will not die along with them. The religious leaders and the Roman leaders were becoming more obvious about their displeasure. There were places Jesus and his disciples could not go with out physical harm and ridicule. Sacrifice Jesus and you can save the disciples and yourself. Hand him over and the valcanoe wont erupt, the SS wont bust down your door and drag you off. Self preservation- a very human instinct. Judas killed himself in the end. If anything, I think that shows he was facing a very personal internal battle that affected the outside world in ways he did not expect. He may have only believed Christ would be put in prison- not killed. We can't say. But then again when we wrestle with tough personal decisions, we usually do not see the full consequences until they bury us.