Yes, a concealed carry would be effective against a mass shooter. Mass shooters usually shoot places where they know there is little to no resistance. A person responding to an incident immediately will throw them off. Make your shots count, if you don't more than likely the shooter will become more erratic and dangerous. However, during the first few moments you will have the advantage of surprise.
I will never give in to something that goes against my beliefs. I will die before I fold, because if you can't keep true to yourself what's the point of being a man.