Presenting a knife and asking for your money is not justification to kill two people, or even one.
The OP asked about the
legal ramifications of shooting
two people, and I think Peetzakiller offered some worthwhile thoughts earlier.
I disagree with your assertion that one is not justified in using deadly force against a robber with a knife, assuming that he is close to you.
That does not mean that I would do so except as a last resort. If he's close enough, my gun will be in my hand, and if he rushes, I just don't see an alternative to firing.
However,
depending on the situation, the opportunity
may present itself to toss a weighted money clip with, say, forty+ dollars in paper money his way while telling him to get back. If what the perp is after is money to buy drugs, gasoline, or food, that and the ominous little red laser dot may save the day. And the doomsday option remains if that doesn't work.
That's a lot more economical than getting into a situation in which you could be forced to come up with $25,000 to $100,000 in fees for lawyers and expert witnesses. And that way you'll still have your gun.
Even in states where you are not required to retreat, why in the world would you risk everything by shooting someone unless it was absolutely the last option possible?
For many people retreat is not reasonably possible anyway, but you are right about the risk. Suppose you fire and the man goes down. There's the evidence: (1) forensic evidence, including bullets and wounds--all that does is establish that you have shot someone, and it
could work against your account of the incident; (2) the knife--that doesn't prove your story, but it
should lend some credence. Perhaps there are witnesses; suppose the witnesses say, "I heard two shots and saw a man with a gun and a man on the ground." Won't help much. Maybe there was a security camera--perhaps helpful, perhaps not. There's your testimony--what makes that credible? Ever been cross-examined?
The point is, you do
not enter the courtroom wearing a white hat. You have shot someone, and it's up to you to present evidence that your actions were justified. They may have been, but that's not enough.
And if the perp survives? There's his testimony-and you would probably be surprised how those guys clean up. I've been on juries, and one cannot distinguish between the good guy and the bad, or between the truthful and the liar, without evidence.
Now, maybe it won't go that far and maybe it will--but you will forever remain at risk from the criminal standpoint if it does not. And then there's "the rest of the story", and that's all downside.