No, I don't think that's all they do, but have you ever seen Generation Kill? Look it up, its on Netflix I believe, aside from the fact that its based off a nonfiction book and aside from the drama like crap that takes place there try watching it and see what you think about the marines afterwards. Its a little insight of how we treated our marines at the turn of the century; there was even a scene, and note that this is based off a nonfiction book now, that they actually rewarded a commanders mistake of misdirecting an airstrike on a populated town. The man hadn't known where the attack was really going to be because he punched in the wrong coordinates but afterwards he was approached, and even though he expected to be scorned for his mistake, the officer came up and gave him a pat on the back. I know its not the best point to make here, but then tell me; when have the marines been called to action that haven't been given the authority to take lives when deemed necessary? I can't think of many, in fact any, at the moment the most I can think of.
When does the military not kill? Well you got the evac of people out of dangerous areas. You got the air dropping of humanitarian support. You got just defending a position, often the military can prevent an attack just by being there.