"We sleep soundly in our beds at night because rough men stand ready to do violence on our behalf." -- Winston Churchill
When it comes to committing violence, most of us are "virgins studying sex." And just as virgins can write about sex, we can write about violence without ever having done it -- and we're far less likely to be called on any mistakes we make in the portrayal than a virgin is for writing unrealistic sex scenes.
It's my opinion that we owe it to those who serve in uniform to portray violence, and the toll it takes, as honestly as possible.
Grossman covers all aspects of institutionalized violence: the deep-rooted instincts that prevent men from killing (yes, we're wired to be violent but deliberate killing is not part of the deal), how factors like physical distance and psychological distance contribute, the role of training/re-programming, the even more important role of group social dynamics, and the psychological trauma that 98% have to work with -- successfully or not. It turns out that about 2% of the population is capable of killing without the usual difficulties, interestingly enough.
What I took away from On Killing was that I was asking my knights to engage in the most mentally difficult form of violence -- short-range, face-to-face slashing and stabbing -- and that what would let them sleep at night was their ability to define those they killed as sub-human, monsters, or otherwise unworthy of living, and the support and approval of both the people in authority over them and those they loved.
Once you look at it that way, a lot of things about soldiers, propaganda and veterans change their meaning.
Grossman also wrote a follow-up: On Combat, which goes into great detail about the physical effects of combat-related adrenaline rushes, panic, and the importance of training. He also goes on at length on his opinion of violent video games, which was less useful from a writer's POV but the rest of the information is well worth it. I blogged about On Combat here.