"No good deed goes unpunished".
IMO the matter of whether or not armed civilians should intercede or not isn't merely one of tactics, ethics, morals, situational parameters, or civic responsibility.
In today's world, to not recognize the price that you may pay -- even in cases where you did the right thing, the right way, at the right time -- is naive.
You can do everything "right", (tactically, ethically, morally, etc.) and still pay a huge price.
As one example, some people need a security clearance to perform their jobs. Any arrests will result in that security clearance being, at least temporarily, suspended pending investigation. If you are involved in a shooting incident as an armed civilian, there's a better than average chance you'll be arrested. You'd be wise to hire a lawyer. You'll be needing some time off work. Expect to lose your security clearance, because one of the requirements for security clearances is that if you're arrested you have a very limited amount of time to call your agency and notify them of your arrest. After you lose your security clearance, you're potentially looking at months off work, while a re-investigation is launched - not by the police, but by the US Office of Personnel Management (and other agencies) who investigate and authorize federal security clearances.
So - you can do everything tactically and ethically correctly in what appears to be a righteous shooting, and you're still out nearly ten thousand dollars in lawyer's fees and you've effectively lost your job. (And that's if it doesn't go to trial.) And this is even before a decision has been made by the DA regarding whether you'll be charged or not.
Everything you did or failed to do during the few seconds of a gun battle will be scrutinized in both the calm, cold light of the DA's offices, and in the media - which might not proclaim you the guy who did the right thing.
And lets not forget that you've now potentially opened yourself up to civil suits as well, from the "victim" you shot if he survives, or from the deceased's estate if he didn't survive. Depending on the whim of fate, you could lose your savings, your home, and a whole lot more.
Oh, and of course the reputation you now have among your family, friends, and community, as the person who killed someone - right, wrong, or otherwise.
Moreover, there are good samaritans who did the ethically correct thing to aid those in need, who subsequently wound up in wheelchairs because they stopped bullets.
When you stop to consider all the ramifications of interceding in even a shooting that is morally or ethically justifiable, you might conclude that you're in fact not a sworn LEO, you are not held to a higher standard, and you are under NO obligation to ruin your life to respond to a situation you neither asked for nor are trained to respond to.
Morals and ethics are for philosophers.
Legally armed civilians would do well to ponder just exactly what they will intercede to protect, when the cost of interceding could be the loss of nearly everything they've worked for all their lives.
Just MHO. YMMV.
Treat everyone you meet with dignity and respect....but have a plan to kill them just in case.