Some of Red Mist/The Motherfucker's nastier moments from the comic, including murdering children and rape were omitted from the film. Christopher Mintz-Plasse 's reaction to this was literally, "Oh, thank God".
According to Chloë Grace Moretz, a sequel to Kick-Ass 2 would be unlikely because it was one of the most pirated movies of the year, leading to a disappointing box office. However, Matthew Vaughn, director of the first movie, has indicated that he still has plans for a Hit-Girl prequel, and a third Kick-Ass movie.
In the wake of the Newtown Elementary School shooting in 2012, Jim Carrey refused to take part in any promotion campaigns for this film, because he felt he could no longer support its violent content. Carrey is known for his personal distaste for guns and gun violence, not unlike his character in the movie (the only gun the Colonel handles in the film is unloaded).
Mother Russia has a tattoo on her left arm, which you can see right after the scene when Motherfucker tried to rape Night Bitch. Translating from Russian it says 'Don't fear, don't beg, don't believe'.
When The Motherfucker is destroying Colonel Stars and Stripes' lair a rarely used 49 star flag can be seen. After he says "Let's trash this place!" you get a clear shot of it. The flag was used for less than a year after Alaska joined then Union in A.D.1959.