As a staunch anti-imperialist myself, I of course denounce the military industrial complex and the horrors we have committed as a nation at the behest of the multi-national corporations attempting to acquire new land for development or resource exploitation.
That said, many of the veterans I have met (and this may just be because of the bias of my own experience and people I associate with) have come from backgrounds that either left them with little other prospects to afford a higher education than a military scholarship, or grew up in families that supported imperialism to such an extent that they felt it was their civic duty to fight on behalf of our nation. Of these people, all of them have grown disillusioned when actually interacting with the people overseas and learning first hand the nuanced experience in a way that none of us civilians would be able to articulate, even if we were to travel abroad ourselves. All of the ones I know who have been in direct combat have stated how they regret what they had to do to stay alive in a situation where they would rather have talked it out. They are the ones who first hand could verify that we are fighting not just statistics but actual people who are driven through ideology or necessity to strike back against our troops. Upon returning home, they unanimously (the ones I know personally at least) have been driven from their conservative and/or centrist beliefs to strongly anti-war leftists.
Unfortunately our government is still doing its best to keep the pipeline to joining the military open, but fewer and fewer recruits are swayed each year. Hence this year they have announced a recall of all veterans under 50 years old to mandatorily re-enlist, even those who have since become jaded and realised what a stuffshow this all is.
All that is to say, the question of people "voluntarily" joining the armed forces is much less black and white than you are saying.
Except for PMCs.
Anyone who joins Academi/Blackwater or the like is absolute scum and just doing it for the pay check and to look cool getting all the top-end gear.