To be fair that scene you posted is a fantasy of the character. Most of the violence in Kick-Ass 2 won't be a fantasy.
I'm still perplexed at why he is taking this stance. The only thing I can think of is they still have that one thing we (who've read the book) all thought they took out.*
That's the only way I could imagine that Sandy Hook would have any impact on him relating to the movie.
*I'd use spoiler code but I forgot how to, plus anyone who's read the book knows what I'm talking about.