It varies from state to state, but...
generally speaking, the Good Samaritan laws hold you blamless for giving aid (to an accident victim for example), as you are "acting in good faith". There is an exception (at least in some states), and that applies to people with EMT training. Those people are required to assist "to the level of their training".
In other words, if you are an EMT (or a Doctor or nurse), you are required to give aid, and can be found liable if you do not. At the same time, the phrase "to the level of your training" protects them from fault, if more help is needed than they can give. Example: Off duty EMT finds a wreck, gives aid, but injured person dies because EMT could not do emergency surgery. EMT cannot be sued for not saving injured person.
We never think this kind of lawsuit would happen, but it must, because the protection was written into the law to prevent it.
As far as I know, this does not apply to CCW holders. States have laws covering the use of deadly force, and many states allow the use of deadly force to prevent certain crimes, BUT no state requires it. It is indeed a slippery slope discussion, but there is a fence at the edge of the slope. You can choose to render aid, but you are not required by law to do so (generally). Not sure how it would play out if you use a gun to "render aid". Depends totally on the situation, AND the location.
There was a case around a decade or so ago (maybe longer, I can't remember exactly), where a guy was on his way into a mall to buy some ammo (he was going shooting), and witnessed a man shoot a woman on the sidewalk infront of the business. The man then shot her several times after she was down. The "good guy" was still in the parking lot, and went back to his car to get his gun. Bad guy gets in his car, and drives away, driving by good guy. Bad guy points gun at good guy, good guy shoots. Bad guy drives out of parking lot, turns corner, runs into phone pole. Bad guy is dead. Good guy goes through the legal process, and while the shooting was justified, he winds up being charged with "usurping the authority of the state", because he acted as both judge, jury, and executioner. This was in Texas.
Use your best judgement, but don't go looking for trouble. If you do, you will surely find it!