Your immediate use of deadly force may accomplish that. On the other hand, it may cause that innocent fellow man to be killed or injured when that might not have happened but for your actions (yes, the drawn gun probably justifies your use of deadly force, but there's reasonable probability that it would not have been fired). No, you would not be criminally liable for the clerk's death, but if you shoot and the clerk is killed, you will never know whether it would have happened otherwise.Posted by FLAvalanch: It's about preserving the life of an innocent fellow man.
It may also cause others to be killed or injured, bad enough without the attendant liability. It may result in your death or serous injury.
If you shoot him in the heart, he would still have an adequate blood and oxygen supply to his central nervous system to empty his magazine, likely hitting the clerk and possibly others including you. From what I've read, two shots to one lung would not stop him instantly. If you empty your magazine quickly, you stand a chance of shooting someone else.
All of that could happen, and if he has an accomplice, he may open up, too.
No matter what happens, if you survive, you will be entirely on your own, with no community to defend you or to pay your expenses during the investigation, pay for a legal defense, compensate you for your likely loss of income, defray medical costs should you be injured, or to pay civil damages assessed against you.
If you do not fire, he may or he may not injure the clerk.
If it becomes an active shooter situation, you and others a clearly in great danger. If my wife were present, I would not consider escape a viable option, but otherwise, I would not rule out escape if it were possible unless it were plainly obvious that I could stop the perp effectively with very little or no risk of hurting someone else.