I'd seen "why you" in a couple of translations, so at first I thought it was just an odd error. Then I heard it in Batman Arkham Asylum, where an inmate said it when Aaron Cash refused to tell him what he wanted to know. From the way it's used, it apparently conveys frustration or something, usually in response to someone who is stubborn and keeps annoying you, probably something like "Why do you keep doing this!? Stop it already!".
Is it a valid sentence people actually use? Or is it just "why you...", in other words just an unfinished sentence? If that's the case, where's the expected auxiliary (are/do)? I have no idea where even to begin to find out more about this; Google search has returned nothing but pathetic irrelevant results.