An ordinary citizen should be no different from a law enforcement officer in this particular. Use a firearm against someone and expect questions.Can you shoot a mass murdering suspect? Of course you can...but your decision will ultimately be questioned and could, God forbid, be wrong.
That said, AndyC had the correct answer, both his first and this, his follow-up.
If you don't act when you need to, your options and your odds of prevailing evaporate.Shoot them in the head, the back, whatever it takes to drive them to the ground and make them harmless; warnings don't come into the picture with a mass-shooter, you just drop them where they stand.
The problem with most people is that they'll hesitate when it comes to it - unsure of their legal standing, instinctively unwilling to take a life due to our Judeo-Christian upbringing where we're taught that life is held sacred and bedevilled by our sense of fair play, particularly impressed into us by old Westerns. We desperately want to do the right thing, to know that we're the good guy before pulling the trigger - and the only practical way to get past that is to prepare yourself mentally beforehand for the awful reality because if you freeze up, you'll likely die along with others.
The best advice I have is to inoculate yourself: think it through beforehand, roleplay scenarios in your head while keeping all the relevant factors in your mind (moral, legal, physical actions, etc) and make sure you're operating on a bedrock of right before that day comes. We're all the first line of defense - act like it by preparing for it.