Some scholars think that Judas' crime was not just that he physically led the Romans to Jesus but that he divulged insider information about Jesus' teachings. For example, Jesus never refers to himself as the "Messiah" or the "King of the Jews" in his public teachings (usually just the "Son of Man"). All throughout the Gospels he is careful to walk a thin line: he is very good at staying out of explicit trouble while preaching a dangerous anti-establishment message. Perhaps in public he limited himself to topic related to social justice (e.g., Sermon on the Mount), healings and other miracles; but among the inner circle of 12 disciples he taught more apocalyptic themes, and alluded to himself as the Messiah or king of the Jews. When he is on trial before the Sanhedrin and later the Romans, these two titles are exactly what he is charged with. Could it be that Judas is the one responsible for revealing this information? Just a theory, but I find it quite convincing.
In my opinion, more interesting than the question "what really happened?" is the question"what is the significance of events to the author/reader of the text?" The point is that Judas is a symbol for the rejection of Jesus by the entire Jewish ("Judaic") people. As we know, the gospels were written a few generations after the events they depict. By this point (circa 80 CE), the Jesus Movement had been pretty strongly condemned within rabbinic Judaism. Remember all the nasty things the rabbis had to say about the "minim" in the Mishnah? By this point, it was pretty clear that rabbinic Jews were not going to convert en masse to become messianic Jews. Instead, the Jesus Movement (by now we can call it "early Christianity") would have to abandon the Jews altogether and invest itself entirely in the Gentile mission. So Judas becomes a lesson to the Gentile audience hearing the gospel - this is why the Jews are no longer the chosen people. It's not because they crucified Jesus (obviously the Romans were the only ones able to do this); it's because they stubbornly failed to accept Jesus as the Messiah and therefore facilitated his death.