Wasn't there a similar situation just recently where the person who stepped in to mediate (with a weapon) was shot and killed by police?
If in the situation described, and I was compelled to intervene, I would likely do so without a weapon, after I had called 911. I don't mean to be selfish, but my days of pulling a weapon to resolve things are more than likely limited to protecting myself or family members from immediate severe bodily injury or death, kidnapping, or rape. In my State, the use of deadly force is justified to protect another person if you believe the intervention is necessary for the protection of the other person and that there is reasonable fear that serious bodily injury or death will result. Keep in mind that the law will judge your actions, and "reasonableness" is the lens through which 'the system' will determine if your actions were necessary. Remember, if you shoot someone, there will likely be someone out there trying their best to convince someone that you were wrong in doing so. Remember, prosecutors, juries, and judges are nothing more than human beings like you and me, and they/we may have very differing opinions on what "reasonableness of using deadly force" really is under a given set of circumstances.
Rest assured, if you pull and/or use a firearm, you will be needing legal defense, and you will need it somewhat promptly.
This whole thing is a sticky one, and when people beyond yourself and family members get involved, it gets even more sticky. I guess the bottom line is that many of us carry in our pockets something that will stop evil people from doing evil things, but remember that it is your story that must convince others that you were right in doing what you did.
Best advice I've likely been given for a situation where you have to use deadly force is "lawyer up, and shut up".