As for wanting to kill, man, that could fill a thread on it's lonesome. Suffice to say, I'd much rather have an army of people willing to kill for their country than an army of people willing to die for it.
If you're willing to kill other people for your cause, you have to expect people to try killing you in return. It just doesn't make sense not to.
Oh, and there have been serial killers who were military veterans before. War
fucks people up. I thought that was obvious. Vietnam veterans had boatloads of problems coming out of there. You also have to consider the atrocities that some people wind up committing
during a war. The military (and sometimes individual soldiers) gets away with a lot of stuff sometimes, and often a wartime environment isn't exactly going to improve someone's sense of the value of human life, especially if they're there to kill people and dehumanize the opposition (which is often a requirement, or at least helps, for being okay to kill them).
I would view joining the military with much greater regard if the US military weren't currently doing what it's been doing, which I simply can't support or respect. Of course, I don't blame the soldiers for that, especially considering how many people join up because they aren't sure what they want to do with their lives, and how brutal and opportunistic their recruiters are.