You can join the war and not kill people. In fact it's very common that soldiers fail to pull the trigger on a live target. For most soldiers, in most battles, it's rather unlikely that they will (knowingly) kill someone, or that they even get in the situation where they could.

I can make up all kinds of moral reasons on why I might object to being part of a war, but "I don't want to kill people" is bullshit. "I don't want to put my ass on the line" is a good reason, it just doesn't sound too good when everybody else is doing just that.