War is extremely destructive and consumes massive amounts of resources that could be better spent. Peace on the other hand allows markets and trade to occur.
Survival of the fittest. The weak need to be weeded out, for the good of the species. However war does carry a lot of collateral damage. I would say it's sort of a break even kind of thing. However, if it weren't for all the wars and death in the past, taking into account exponential growth, we would be way more over populated than we are now.