Bingo!
That brings to mind the story related by Massad Ayoob about the retired Marine who was ordered to the back of the store during an armed robbery. Read about it here.
In most jurisdictions, the use of deadly force to prevent a forcible felony such as an armed robbery is justifiable.
The issue is one of a reasonable belief that the use of force was necessary and not excessive. The actor would be responsible for his or her own expenses in the event of an investigation or further proceedings.
Yes, of course, and if it were argued that the actor had failed to take into account readily foreseeable risks (say, by firing an unaimed barrage or firing when third parties were known to be behind the target), charges of criminal negligence could enter into the picture.
A friend of mine who served in a major police department for many years and who participated in investigations and trials involving police shootings and the attendant the civil liability of his municipality, can be expected to emphasize, whenever a discussion such as this one comes up, that he is not about to put everything he owns at risk to defend anyone but himself and his family.
Personally, I'm willing to extend that circle just a little to include others whom I know well if necessary, but I have not seen what he has.