Many states have a section in law that allows for (protects from prosecution)the use of deadly force in defense of others, but it is not usually found in the laws covering CCW. IT will be somewhere in the laws covering murder/assault, etc.
Good Samaritan laws are legal protection for those who honestly try to help and unintentionally cause harm. It is generally applied to medical/emergency situations, such as car accidents. It may be applicable to a defense shooting in protection of othes, but I don't know this as certain. Contact a lawyer in your state for a valid opinion, please!
No ordinary citizen has a legal obligation to intervene with deadly force. Your personal moral/ethical obligation is diferent. The law protecting you from prosecuton should you use deadly force in defense of others does not require that you do so. Having a CCW does not deputize you. It does not make you an unoffical law enforcement agent. It does not require you to come to the aid of others. And, should you do so, you better have clear cut, obviously legally correct circumstances, and you actions better be the minimum necesary to protect life and safety. (see the thread of the "former hero" who is now in deep kimchi for chasing and shooting at some shoplifters)
If you are ever in such a situation, you will have to make decisions between hearbeats, and a wrong decision can put you in jail, or worse. So think clearly about it, beforehand. You cannot legally use deadly force to protect someone else's property (or your own, usually), only to protect their person from threat of or actual grevious harm. It is a gray area is some apects, but in others it is quite clear in law. Be sure of your ground, and act accordingly.
All else being equal (and it almost never is) bigger bullets tend to work better.