"we kill for all sorts of bullshit excuses"
Bullshit excuses are what puts boots on the ground. People can get behind a bullshit excuse like "those people are ethnically inferior" or "those people posses weapons of mass destruction and will use them without discretion" or "they worship a different version of the same god." What you can't get people to support is an idea like "we need war for population control." Because historically, that has been the driving cultural factor (citation, Marvin Harris). Ultimately, the extent to which we do war is negligible because war is just a devastating way of undergoing some type of change within a nation/state/people group, and is therefore going to be a constant setback of civilization:
The US invading and disrupting multiple countries in the Middle East is predicated on the cultural need to prevent an organized Islamic empire from challenging the US as an international hegemon (and meanwhile, the American public feels uncertain as to why we participated in those wars at all...). We have other methods of doing so, like supporting the existence of an Israeli state and like imposing economic sanctions on Iran, but the underlying issue is that a reunified Ottoman Empire would pose a huge thorn in the US's side. (citation, George Friedman and STRATFOR)
Another easy example is Russia's continued presence in Ukraine and their continued (although not as publicized) pressuring on other Eastern European states (like Belarus) to loosen their economic independence in favor of stronger ties to the Russian Federation. Putin may say that he is doing this to support ethnic Russian populations, or to strengthen Russia's (and neighbors') economy, but the underlying cause is that there is increasing pressure on all sides of Russia from ethnic minorities (like Chechens and the Sakha Republic) that are pushing the Russian Federation towards collapse. In order to protect their own interests, Russia must expand west or else face the historical alternative - Europe expanding east. Whether Putin knows that Russia faces an existential crisis by not pushing west is unimportant. The fact is that the Russian people will expand west under any guise necessary because the operations of culture are guided by an invisible hand. If Putin tried to pull out of Crimea, he could not stop the ethnic Russians in Ukraine from trying to join Russia. It doesn't matter what any leader does to his nation, really, the world will adjust and do what is culturally necessary. (citation, George Friedman again.)
So, the fact of the matter is that if we didn't go to war, there would still be some kind of cultural pressure to resolve whatever factors lead to the war. Whether that be some form of racial tension, mere population control, the enforcement of a policy of industrial expansion, or an attempt to abolish slavery and increase federal authority. Go ahead and challenge me on this. I've bought into this theory, and I'd like to practice defending it...
Oh, and yeah, it really sucks for a lot of the individuals affected by war. But in all seriousness, it always is intended for the good of the nation (nation referring to people of national identity, not the governing forces of those people). And as George Friedman put it, governors play their parts like in a potential game of chess where people rotate out each move. Each man's one move is strictly limited to the environment he is introduced to. There's only one best move he can make, so there's really no free choice in what he does. A Republican leader will just continue whatever the guy before him did, and if it were a Democrat in charge, he would do the same things, but give them different names. A true assessment of politics starts with a study of nature-culture.