I'm assuming the drug deal qualifies as a felony
Distribution & trafficking of drugs is usually a felony. Possession, which is all that is implied by attempting to make a purchase, is generally a misdemeanor (most states).
It seems to be significant that the person in question was a would-be purchaser, not a seller.
The notion that trying to buy pot is a serious enough crime as to nullify the right to self-defense is farcical, absurd, and obviously unconstitutional (no due process, also death seems cruel & unusual punishment for a misdemeanor).
Once the robbery began, the commission of the crime ended, as nobody still intended to buy pot. It was merely one person defending himself against a robbery at gunpoint. This has about as little to do with someone getting shot during a bank robbery as if I go take a giant **** right now. Although the law is worth just as much, and smells just as bad.
This is not a case of someone being killed in the commission of a violent felony (which might reasonably constitute a crime, complete with an
actus reus and
mens rea). It is a case of the law depriving certain classes of offenders from exercising their right to self-defense without any sort of due process. It is abhorrent, reprehensible, disgusting, unconstitutional on its face, and I will say the same about anyone who supports such a law (minus the unconstitutionality).