V for Vendetta was more about Padme, the curator guy, and the main villian than it was about V.
He can't be killed because he has an incredbily high regenration factor. He's been turned into goo/slush and came back alive shortly after. He's been decapitated and came back to life shortly after. He's been full of bullet holes and he ran around laughing about it. A movie about him, if it was true to his origins and powers and what not, would be boring cause theres no tension. Theres no "oh man, he might get ****ed up" because he just comes back. It's like Wolverine in the X-Men movies. Once they showed him getting shot in the head then getting back up it was over. When it's established that a character can, essentially, cheat death then that charcter generally becomes boring/stale as most of the things they face are about as much thread as a cunt hair.