It has often been said that the remedy for a bad guy with a gun is a good guy with a gun.
Yes. That is why we arm our police officers.
They have open carry in El Paso, and at the Walmart where the atrocity happened it was not uncommon to have people carrying long rifles. But law enforcement has expressed concern that a number of people carrying weapons complicates and confuses their entering the scene.
I'm sure it does.
These problems can be reduced by properly training people as part of the concealed carry licensing process.
The problems can't be completely eliminated however. There are cases where even police officers mistakenly shoot each other.
Do you see it as mostly true that it's best to have a lot of armed civilians (good guys) at the scene when a gun atrocity occurs?
Yes. Leaving people defenseless in the face of a massacre is much worse.
Although our right to carry guns would trump everything even if guns were a negative.
That actually poses a dilemma in how best to reply when someone claims that guns are a negative. Do I argue that they are not a negative. Or do I argue that we have the right to carry them even if they are a negative.
Both points are equally vital, but focusing on one of the points tends to minimize the other point.