This is just my personal opinion born from my little observations about the world so I could be completely wrong but...

I believe we are geneticly programed NOT to be able to kill each other in order to protect the species from self-destruction. From what I understand animals in general don't kill members of their own species except rarely and accidentally. Since I believe evolutionism I believe humans have the same instincts as other animals. Most normal people would have a hard time killing another human in normal circumstances.

As an example: as far as I know soldiers, for who killing is "good", need to be conditioned to be able to kill. It's not something natural in most cases. And in order to make it easier the "enemy" is always demonized and made to look non-human in the eyes of the soldiers. And at the end of the war, from what I have heard, many soldiers have long term psychological problems.

Again, I have little information too back this up so I could be wrong. It's just from my observations about people in general.