I can see the argument that by taking the risk of hitting an innocent bystander your attempting to eliminate the threat before he can do more damage than your single errant round might.
Here's a hypothetical for you though. You take the risk and fire two rounds, the first kills a young child that ran through your field of fire as you pulled the trigger. The second takes out the gunman. When police examine his weapon they find that it has a broken firing pin and the round he fired 2 seconds before your first shot was the last he would have been able to fire.
Will the parents of the child agree that your action saved lives or not?
I seem to recall hearing that the gun of the gunman in the theatre in Colorado did indeed jam. Did he have the skills to fix the jam and continue killing or was the malfunction the cause for his exit of the theatre?