One popular claim is that, while it's easy to try and kill someone who threatens you, it takes real nerve and resolve to not defend yourself. This isn't the most ideal source in the world, but there's good reason to doubt the idea that the willingness to kill comes easily to humans.
It's not always hard. And it's not always easy. In our culture? Passivity, non-violence, even being a victim, is held in esteem. It is praised and treated as superior, particularly where Christians are concerned.