This was a tough call for Warner Brothers to make and I can't blame them in the slightest - GANGSTER SQUAD, which featured a movie theater shootout promiently in the film's trailer, will undergo edits to remove the scene from the film and any subsequent trailers, according to Variety. The movie will have to undergo quick reshoots to change the movie, as the sequence was important to the story.
It is very likely that this will cause the film to be moved from its September 7th release date, and hopefully the movie will be able to work without the scene. Ordinarily, I'm not for editing movies in this fashion in reaction to outside events, but in this case, I think it's appropriate. I'm certain movie audiences at this juncture would not like to be subjected to a scene like this; at the very least, it would be considered in very bad taste. Whether or not this affects the quality of the movie is of course for audiences and critics to judge upon release, and hopefully Ruben Fleischer's movie won't suffer too greatly due to this new edit.
EDIT: Many talkbackers (and Billy the Kidd) have said that it makes more sense to just move the movie a few months down the road. That makes sense to me. I don't think there will ever be a point where people won't be reminded of Aurora when it comes to this sequence, but perhaps when emotions aren't so high, the movie will play better to audiences. It is a gangster movie, after all; people get shot in gangster movies.