that would be a stupendously fallacious argument to make on their part, and would have immeasurable ramifications on the law if they tried and, God help us, succeeded.
the first thing you get hammered into you in any CCW class worth its salt is that your foremost goal is to GTFO. you aren't there to secure the location. you are not there to fight criminals. your job as a civilian is to GTFO and keep yourself safe. when i took CCW class way back, two things were said to me upfront: you CANNOT draw your weapon and open fire unless you and everyone else involved have 100 percent certainty of the situation and know everything going on, and have a reasonable belief that your life is in immediate danger and you have no other options, and two, you focus on yourself, call the authorities, and get out. a video clip of a gas station robbery was shown in my class, and we were all asked what we'd do if we were browsing the merchandise and saw the guy with the gun mugging the cashier before shooting her in the chest - she died.
some said they'd draw their gun and kill the robber right then and there. that's the knee-jerk reaction. but, you still don't know the whole situation. the apparent bad guy could have been working with someone else and that accomplice would take you out. if you kill the apparent bad guy without full knowledge of the events, you can be nailed to the wall for it, especially if the shot was made without knowing all the variables. if it was a fake robbery intended to commit some kind of fraud, and you killed the guy, you and the cashier would be facing the same severe punishments.
of course, the vast majority of gun owners don't get training. they buy a gun and often never use it, which is why there are so many weapons getting into the hands of bad guys.
rosenbaum wasn't murdered, and all footage leading up to his death shows he was indeed fucking around to find out. even if kyle did murder him, and if it was proven that he committed homicide with deliberate, malicious intent, the question must be asked what the fuck is anyone else doing chasing after him and accosting him, as we've seen re-run plenty of times? grosskreutz, in a sane world, is already being pilloried for his actions that night, which he recorded himself: he gave chase to a supposed active shooter and agitated the crowd to attack him, leading to more self-defense killings. kyle was not an active shooter, and the results speak for themselves: only two people died and lefty was maimed, instead of dozens of bodies lining the streets of kenosha.
not a single cop was there to do anything about any of these shootings until after the fact. the reality is they were all held back by democrat leadership explicitly so these bodies would pile up.