Dave Grossman is outstanding on the psychology behind killing, but he seems to have some hangup with video games.
I think I see how he gets there, but I think he's very badly confused about the issue.
First, Grossman's observation that the vast majority of people, if simply handed a firearm, and some technical training and marched off to war will fail to use their weapon effectively, even in the face of imminent peril is well documented.
Second, this reluctance can and has been effectively been overcome with conditioning techniques - this is now a primary focus of bootcamp.
Third, this conditioning can take several forms, of which video games are one method.
What Grossman fails to grasp is that video games in isolation have never been used for combat conditioning, but only in conjunction with other training and more, that they have not been used alone because they are insufficient for most people.
What is likely, though it probably has not been sufficiently examined, is that other forms of conditioning are probably sufficient in isolation among the same subset who find violent video games and movies adequate to overcome the natural aversion to killing.
Hunting for example.
Bless, O Lord, this creature beer, which thou hast deigned to produce from the fat of grain: that it may be a salutary remedy to the human race, and grant through the invocation of thy holy name; that, whoever shall drink it, may gain health in body and peace in soul. Through Christ our Lord. Amen