Since you're asking about historicity I will be dealing with the canonical Gospels here as they're closer in time to the event and leave the "Gospel of Judas" out.

Judas is a difficult character to work out, especially from a modern moral point of view. He betrayed his master, which was an abhorrent crime. But in doing so, he apparently saved humanity. For the theological and moral implications of this, I'll just quote Bob Dylan - "I can't think for ya, y'have to decide//Whether Judas Iscariot had God on his side".

Often, however, in source criticism, such narrative discrepancies are taken to suggest there may be more to the story. So let's see what happens... Mark 14 begins with the Sanhendrin plotting to kill Jesus, and they agree that they do not want to arrest him among the crowds 'lest there be an uproar'. Judas approaches them, agrees to betray Jesus and is promised money. We're told that Judas then "looked for a convenient way to betray Jesus". Eventually, Jesus and the twelve retire to have the last supper, and go out to the Garden of Gethesmane. Judas shows up with "a multitude with swords and clubs [that] came from the scribes and the elders", and delivers his Judas kiss to identify Jesus. Jesus is arrested, and the high priest asks him if he is the Christ. Jesus says that he is, and he is accused of blasphemy.

Eventually we move on to Mark 15, where we have perhaps the most interesting scene: Jesus faces Pontius Pilate, who asks if Jesus is the king of the Jews. Jesus replies "So you say yourself".

Wait, what?! Jesus never calls himself the King of the Jews... right? And instead of replying "I am," and prophesying as with the Sanhendrim, he plays ill-advised word games with Pilate.

Well... let's turn to the gospel of Matthew, 19:28:

Jesus said to them, “I tell you the truth: In the age when all things are renewed, when the Son of Man sits on his glorious throne, you who have followed me will also sit on twelve thrones, judging the twelve tribes of Israel. 

If Jesus is their Master, and they are judges of the twelve tribes, what would he be if not King of the Jews? The passage is also found in Luke, and there are oblique allusions to royal status of the disciples in e.g. Mark 10:35-42. And... one of the people Jesus says this to is, well, Judas Iscariot himself. How can we make sense of that?

If Jesus said this, it may explain one oddity in Judas' betrayal - how hard would it be for the Sanhendrin (Mark is very careful to emphasize this, it's the men of the Sanhendrin!!) to just follow Jesus and arrest him at a convenient time? What if Judas' betrayal was not that, but instead informing Roman authorities that Jesus intended to subvert them, make himself king, and his disciples judges? Suddenly Pilate's accusation makes sense, and we have to wonder if the contrived Mark 14 is merely part of the long tradition of blaming the Jews. Crucifixion was, after all, a very Roman punishment. And it would make sense if the quote was indeed real. What reason could an author see to invent it? It makes no sense in terms of narrative consistency.

This is, in the end, just one possible interpretation and way of making sense of Judas. There are others. I draw this, apart from the NT itself, mostly from the works of Bart Ehrman; How Jesus Became God should cover it.