I have a hard time accepting that people have a universal nature, I don't like to think that there is inherently something violent and competitive in our natures by default.
But, it could be so.
It is like that in the animal kingdom and even in the plant kingdom. We're still products of the natural world.
However, there are people who transcend this and strive or yearn for peace and collective well-being. So, there is a choice and our destructive natures can be overcome. Based on this, humanity does have the capability to rationalize that war is deadly and wrong.
I think that someday, there will be a consensus on this. Maybe a social evolution if we don't destory ourselves and the planet before we get there. But our civilization as we know it will have to radically change. In what ways, I'm not really sure.
Industrialization has wreaked havoc on the planet in less than two hundred years. Ecologically and socially.
We've advanced technologically in ways that allow us to stay alive (medical) but we've also advanced our weapons.
There have always been wars but now we've got the means to fly thousands of miles just to bomb entire cities (as Sinistrad said) or destroy a nation with a press of a button.
is War nature's way of keeping us from over-population?
Now that's an interesting question. Based on evolution, it assumes that we're born to live and perpetuate genes, that's it. That's the meaning of life. Just to merely exist; that we construct our own personal meanings.
However, there is also a self-destruct mode to keep it in check. If that is natural law, who is to say that this self-destruction is 'bad'? and from where do we define the word 'bad'? So then to live for collective well-being would go against nature and the law of the Universe and that would be 'bad'?
I'm sort of rambling and lost in thought now. I haven't really answered your question. I don't know if I can.