I'm surprised there hasn't been more replies. But you guys made great ones, so I'll comment on that.
This is probably one of the touchiest subjects in the industry... While I don't think it is right to just sit there with a camera, what could the photographer do to help? The lives of the ones being effected are much more important than photographs and as such a photographer should do what he/she can before settling to just taking shots. On the same note the photographer trying to help can also make the situation worse and can injure themselves and the onlookers. At the same time the photographer should never let an opportunity pass up which is why the camera they use should fit perfectly in their hand and way of thinking. The D700 as a body part (second eye, perfect fit, control without having to look) can be much more effective than the swinging box on your neck that you have to thumb around with for every shot. This subject is most prominent in photojournalism where shots are supposed to help in the long run...
No doubt helping out could be an issue, especially if you're inexperienced, which is why I said, take every case individually.
It indeed is a touchy subject, but I'm sure others have come across it in the past.
Have you seen the Midnight Meat Train?
There's a scene pretty much like this. Somebody look it up on youtube.
Personally if I had the opportunity to take the guy out by whatever means possible I would do it within a heartbeat. The guy is stabbing a helpless person. WTF? Am I am just going to stand there like I don't give a shit? Hell no! The moment he let's his guard down, it would be the moment I would use to take him down.
"We must always take sides. Neutrality helps the oppressor, never the victim. Silence encourages the tormentor, never the tormented."
On the subject of being a war correspondent I really don't know. My first instinct would be self-preservation. I would always carry a gun and know who to use it. But if I am in the situation like you described, where we are being ambushed, I would leave the fighting to the soldiers. I would only be an additional risk to them. The are trained to fight in scenarios like these. They fight like a unit, and I am just a guy with a camera. Me going and starting shooting would not help that much. Do what you do best. Leave the shooting and fighting to those who were trained and experienced in it.
I don't know, if I were a soldier and I had to escort a war correspondent, I'd be really annoyed that I have to protect my own rear end along with someone without a gun. Which is why I'd probably get at least minimum training with a gun and learn how to handle it as well, if I were a correspondent.