It may not be wise for me to speak here, as most everyone here is anti-war (which is a wise stance) and few are exactly pro-military.
I did make the decision to join the armed forces when I was 17. As soon as I graduated high school I shipped off to basic training.
As far as killing people ... well. You have to understand that every Soldier, Sailor, Marine, or Airman is tasked with doing work in a certain field. Mine was engineering. Most servicepersons do not shoot at people. On the other hand, all servicepersons should expect that possibility.
I joined out of patriotism and love of my community. Yeah, a lot of you will argue that I fell for propaganda and marketing. Maybe that's so. I did not join to murder anyone. And I admit ... I wondered how I would do if situations got out of control or a war broke out. Of course, war(s) did break out.
I'll say this, though, and this is probably the only important part of my post:
When you end up in a frightening place you aren't supposed to be in, or if you are told to do something very dangerous that isn't even necessarily your "job" to do, you won't be thinking about why the war is even happening. It won't matter at that moment.
What will matter is that you watch your friends' back. That is truly what it comes down to. Most of us are very caring people -- that's what drives you to fight, in the end. I'm a humanist. I love life. But I'm merely human myself, so I want to protect the people I love, my extended family in uniform.
Anyway, let the flames commence. I'm not a nationalist, but I do love my country. I love every country that I've visited (except one, which shames me).
But, yeah, most people in the military aren't frontline killers. Only a small percentage are infantry / cavalry / go outside the wire on patrol. Most are in normalish jobs. Radiomen. Cooks. Mechanics. Drivers. Corpsmen. IT guys. Fire control guys. Electricians. You name it, they have a job for it. Even journalism and photography!
Of course, you don't really get to pick your job. You get to choose from a pool assigned to you at MEPs based on your ASVAB / experience / what is needed at the time. In the end you go where they tell you to go and do what they tell you to do.