That’s not his point.
Grossman argues that videogames are producing a whole generation of potential mass killer; that the exceptions of yesterday are becoming the norms of today.
He’s wrong. I’m sure it plays well to the “let’s ban everything” crowd, though.
Well then, his argument fails on multiple levels.
First, these sorts of shootings are extremely rare, so even if they have increased in frequency, they are not being driven by video games, because we haven't seen an increase anywhere near proportionate to the vast increase in video game popularity.
Second, millions of young men have gone through the same "operant conditioning" as one supposedly gets in a video game when Uncle Sam taught them to be soldiers and Marines, plus many of them play First Person Shooters, yet they don't go on murder sprees.
Third, a large number of mass shootings are carried out by adults who are well past the "juvenile" age.
Fourth, the description of these games as "mass murder simulators" is sort of like saying that Battleship or Axis & Allies is a mass murder simulator. Most FPS games deal with war, and in "mass murder" shootings, nobody is shooting back. It's difficult to trust the conclusions of someone who can't even describe the medium as it actually is.
Fifth, even if we say the argument is that the games are not making the murders any more likely but simply make the murderer more effective, that's a theory, not a conclusion supported by evidence, and certainly not a reason to ban a product when millions of people will use it and never harm another human being.
It's time for us to stop blaming the actions of occult freaks and mentally ill people on people and products that had nothing to do with the crime. It's morally repugnant, and is just the 21st Century version of the whole "Dungeons & Dragons causes suicide" stupidity.