In the book, Grossman talks about how our grandparents' generation was far more likely to have seen dead animals, and perhaps been close to the killing - living on a farm perhaps and seeing or even doing it - chicken for dinner, or the Xmas pig slaughtered for the holidays.
He points out that many people, especially urban folk, are far, far removed from this familiarity and relationship to death, and so don't 'know' it in the same way. Now, I'm not talking gang bangers or crims here, just everyday folk.
So to answer your question BlackOps, no, I don't think this 'natural' aversion Grossman posits, has changed so much, but what has changed is this fantasy relationship with killing from movies and video games, which might lead people to be more willing to pull a trigger, but less connected to what they are actually doing - i.e. taking a life.
I wonder how a random sampling of people would react if given a live chicken and told to kill it? ...Or perhaps given a baseball bat and told to kill a dog, or shoot a horse perhaps?
I grant that the motivation to kill might be less because the chicken is probably not a very great threat to one's existence, but I guess what I am getting at, is if there is any greater willingness to kill nowadays in comparison to 100 years ago, perhaps it's more due to the 'trigger pulling' being separated from any consequences, or meaning in the act itself.
As I remember it, Grossman alludes to this as a factor in PTSD.