Maybe. What if the shooter throws down his rifle and surrenders?
From his first shot fired surrender is not an option, I'm not going to arrest anyone in those circumstances. More than a few lawmen have been gut shot by a hide out piece after a BG has thrown down his visible weapon, and if you've noticed multiple firearms are the status quo for these rejects.
The "vigilante" label would not apply, since I'd still consider dropping the shooter as paramount for my safety as well as the safety of others.
If I only severely wounded him to the point that he was no longer any possible threat (unconsious or with spine severed) I would probably let him live, but one way or the other he would go down and stay down.
This sort of situation is extremely atypical, this is not some dumb kid trying to rob a convenient store with a airsoft pistol.
As noted these Darwin award winners go in expecting to die, so any pretense of surrender is almost certainly bogus.
Besides which. Any engagement would last seconds at most. With any luck at all the first and last he saw of me would be a muzzle flash.
There would be no mexican standoffs or "halt or I'll shoot", thats for the movies.
Whether his gun hit the floor before or after my bullet hit his brain would be impossible to say for sure unless caught on a security camera.