I think the question is kind of moot.
Someone should just ask Mark Millar, Jane Goldman, and others on creative staff whether or not it's a libertarian movie.
If no one says "yes," then you're ultimately just picking out libertarian themes you find that are really present in all kinds of works, especially vigilante genre films.
Kick-ass will be remembered as a superhero vigilante film, just like 300 was remembered as a military war / quest film. These questions over underlying ideology are almost entirely irrelevant.