I read a book sometime last year, the title of which currently escapes me, that examined the subject of human resistance to killing throughout history, and concluded that even in 'martial' cultures, there's a surprising number of people who don't stab the enemy, fire their weapons, etc. In some cases, battles are even set up in such a way as to minimize killing. It becomes as much about display and intimidation as actual 'violence' in the traditional sense.

I'm not completely sure how much to trust those conclusions, but the case was fairly compelling. So there does seem to be some sort of innate resistence to, if not violence, then actual killing, in a large percentage of the human population. Ninja training is designed to overcome this, and while shinobi in other villages may seem not to have problems, you'll notice that in many cases their emotional development is rather stunted, and they seem prone to anger and fatalistic despair.

Eh. As always, YMMV.