And by "this," I mean the idea that art can inspire mass killings in any capacity.

There's the "no fucking way does art do this" school of thought.
And there's the "you're insane if you think art doesn't influence children's development" school of thought.

While I'm sure the answer is generally more complex than either school of thought will accept, I think any attempt by armchair psychologists to get to the core of the problem of a mass murderer is generally futile.

There is absolutely, it should be said, an argument to be made for the male domination of Hollywood and its indulgence in male fantasy helping to shape the minds of the young viewers in terms of their expectations and treatment of women. And I think bringing up certain Apatow films can absolutely help make that argument (Funny People and This is 40, in particular, although they're not really aimed at young people, but I find them generally misogynistic and loathsome). And while I don't think that all misogynists are future murderers (clearly), there's obviously nothing wrong with using a now-famous misogynist to discuss publicly the real tragedies of this school of thought.

However, for Apatow to be your sole name-check is strange considering his role as the producer for Bridesmaids and Girls, two of the biggest and most progressive female-driven hits of the last decade in any medium... and to name-check Neighbors as your film solely because it's the film du jour is stranger still, because, well, it makes me think you haven't seen Neighbors. And when crafting an argument like this, using the proper evidence for your thesis is everything. I'm sure Apatow fanbros would have still fought back if different films had been used, just as fans of Project X and Todd Phillips would have done as well. There are disgusting men in the world who will fight back against any argument a woman makes about unfair treatment of women in any regard, and they should all be taken out behind the shed and Old Yellered.

That having been said, I highly doubt at least the posters I know below are responding that way because they fall under the fanbro category. They're usually, if I recall, the hyper-liberal dudes in GD calling the posters with backward-ass thoughts about women assholes. I imagine they just strongly object to the implication Hornaday makes that art can lead to murder.