Is the premise of your argument really about wars or is it about overpopulation?
If your sole argument for a 'good war' is that its benefit to reduce population size is grounds for defending it, I really have to disagree. I think as far as methods go to combat overpopulation, war is one of the most inefficient ones.
Much more practical solutions to overpopulation could be thought of, such as sterilization (not necessarily suggesting or condoning); implementation of a child policy similar to China's, or, something that is very likely to happen in the future- inhabiting other planets. There is no reason why in the future this couldn't happen.
More and more men are becoming infertile by the day. There is a lot of estrogen in the water supplies these days (completely unintentional); it gets in there from women taking the pill and there not being a filtering process to take the hormones out. Also a lot of plastics use synthetic estrogens, which also effect hormone levels. I forgot which species it was, but one of them lost all males in the period of 20-30 years and instead became androgynous. All of this due to 'gender bending chemicals.' Excuse the turn of phrase.
In fact, the European Parliament has noticed the disturbing effects of all this estrogen and has started banning certain synthetic ones as it's effects are too pernicious.
Male infertility will be problem in the future. How big? I'm not sure. But if testosterone levels are anything to go by, they've been dropping since the 70s and at a rapid rate.
All of these will be factors in your solution to overpopulation. I really don't see a war as being necessary.