You can not use deadly force just to exterminate two pieces of filth. You can't even point your weapon at someone without justification, because it constitutes assault.
You have to be in "fear of death or grievous bodily" injury to yourself or a third party.
The demographics of the victim and criminals come into play, too. Had the elderly victim pulled out a Detective Special and sent these two to their richly deserved reward, I doubt that any grand jury would ever indict her. If a 6'1" 190 pound male witness pulled out his .45 and blew them away, even while they were trying to drag the old lady, he'd have a hard time explaining why he used deadly force in the absence of a weapon. He'd probably go to jail.
The manual modification of the nasal passages and orbital sockets of the criminals to stop the robbery and prevent escape, however, would likely be permissable.
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