He's enigmatic, thus the popularity of his character.
Yes, he fights for personal freedoms, so he can be Good.
Yes, he commits evil acts in pursuit of Vengance, so he could be Evil.
He does both, so he could be neutral.
I think of him as very similar to the Operative from Serenity, though he was fighting for Order, and V fights for freedom. The difference being was that the Operative knows he is Evil because of what he does and admits as much. While V does not admit it, it is not inconcieveable that he could have had a similar speech in the move; it would have fit.
That being said, you can argue for Evil, Neutral or Good.
I'd argue Chaotic Neutral. Some Good intents, some Evil intents, with some good acts and evil acts.