I say, let the guy go home. After all, if you are a Marine you don't want this guy to have your back.

As far as joining the military without knowing he'd be asked to kill, MAYBE. However, after basic, I don't think he could make that argument. You shoot guns at targets that look like people. You use bayonettes on dummies that are shaped like people. More than likely you get a Sgt. yelling at you about how you need to kill the enemy before they kill you.

The guy doesn't believe in killing. That's fine. So on second thought, don't let him go home, make him work a desk job. Make him earn the pay he's signed up for.