Pulling back to explain a bit more of the plot: Dave Lizewski notes that nothing really distinguishes him from anyone else. He has only a couple of nerdy friends (played in very adept comic performances by Clark Duke and Even Peters) and his designs for a girl (Lyndsy Fonseca) are entertained by the object of his desire only because she thinks he's a homosexual and because she wants a "gay BFF." (He also, in one of the film's biggest misplays, a cringe-inducing development, has sexual thoughts about his English teacher.) He is also routinely mugged and stunned by how people refrain from helping, only watching dumbfounded (the quote "Evil can only exist when good people do nothing" comes to mind). He decides to suit up to combat the crime of the city, and soon enough he's widely known as a real superhero. This causes commotion among the felons, such as D'Amico, even though Kick-Ass isn't responsible for a lot of the things that people think he did. Big Daddy and Hit Girl are the ones accountable, going for revenge against D'Amico, who sent Big Daddy to prison through planted evidence and is now peddling a lot of cocaine.
This is both more appealing and more negative than it sounds, well-played by Vaughn and his solid set of actors, including a winning Johnson (who I hope to see more of), Cage pushing too far to diverting effects, and Moretz, who has (perhaps too much) facility. These performers and amusing (if flawed w/r/t narration) writing make it a real pain in the ass that the film goes down the road that it ultimately does. B-