My position is that there is no difference between those in the military or a first-responder occupation and "regular" people. Firefighters and cops have merely taken on a profession where they face these situations all the time, but the moral obligation to aid another is the same when faced with such a situation no matter what your occupation or station in life.
I admit there is a training and equipment factor, some situations that are completely impossible for the untrained or underequipped become possible with the right gear and know-how. I certainly don't advocate throwing your life away pointlessly when there is absolutely no hope. Often direct intervention in situation is tactically unsound... for example, I wouldn't get directly involved in a domestic dispute unless there was no other way to save a life (those things can turn BAD fast)... but if that was what it took I'd take the risk.
In closing I'll mention that I'm sure those blow-hard comments weren't directed at me. I've seen the elephant many times and he is big, gray and ugly.
EDIT: Entering the water in a mass-rescue scenario is at least as dangerous as drawing-down on an armed BG. When people are drowning, or even simply panicing, in the water they try to climb up on anything... including rescue swimmers. Half of lifeguard training is learning how to keep the victim from killing you. One victim is bad enough, now multiply that by the five or six that might be in arms reach, all trying to get ahold of you and push you under so they can stay above water. I'll take my chances with dumbo at the 7-11.
Military/Police/Firemen/EMT scenarios are not what this thread is about - it is about the ordinary Joe citizen/civilian with a ccw that stumbles onto a situation where he may be able to save a crime victim (not an accident victim) from harm, and what obligation does he owe a perfect stranger who hasn't taken the time and effort to arm himself to avoid being a victim. Without a very specific scenario all we can do is generalize.