[alternatives?]

But if I shoot one guy, do the others have a right to be armed and take me down?

[snarkytart]

self defense is reasonable fear of your life..I think after all that was going on and being attacked Kyle’s mindset was definitely one of being in fear of his life.

[Cboldt]

-- But if I shoot one guy, do the others have a right to be armed and take me down? --

That's the active shooter question.

Anybody in the public has the right to use deadly force against an active shooter, assuming the jurisdiction allows deadly force in defense of self AND OTHERS.

Only the police have a right to use force against somebody who is standing down.