I asked a cop about this specific situation on another forum and this was his response to me asking if pulling a gun and using it if needed would be backed up by the law if it was used to possibly save someones life.

"Self defense (or defense of others) is a big grey area, especially when it involves deadly force, and especially if the other person is unarmed. It all pretty much boils down to how you can articulate the situation to the judge after the fact. If you honestly feel in your heart that that man will die if you don't do something, and the number of people attacking him leads you to believe that they would turn and try to seriously injure or kill you if you tried to intervene otherwise, then you do what you have to do."