On making things worse

I’ve seen people claim that having an armed citizen engage an active shooter in self-defense would somehow make things worse.
How? Isn’t an active shooter targeting innocent people without resistance already one of the worst things possible?
I don’t get that thought process?at all. Why would someone not want themselves or others to have a fighting chance in?the event of violent attack? I can’t wrap my mind around that thought process at all.
There’s no guarantees that the armed citizen will succeed in stopping the bad guy, but at the very least the bad guy would be distracted and need to respond to the armed citizen, giving others the opportunity to escape, move to a safer location, or fight back.?Sure, the armed citizen may miss and might hit innocent bystanders,?but the same could be said about police, and those people may well have been intentionally shot by the bad guys regardless.