>He starts with the observation that most men has a natural resistance to kill (only 2% of men

I think that premise is wrong. People easily kill each other in so called affect state, i.e. when they are enraged or afraid of death. I guess he says about 2% only when there is calm person who need to kill someone unrelated to him at all.

>This resistance is the reason why infantry fire was so embarrassingly ineffective in the past 300 years - with the exception of machine guns - despite the fact that infantry weaponry (rifles) are reliable and accurate enough to cause massive losses among the enemy.

There are plenty of reasons why armed warfare wasn't too costly in past times, and most of them are technological. But this is very broad theme to discuss. Avoiding of unnecessary violence sure has relation to it, but putting it forward seems wrong. Although broader concept like "discipline" can include it too.