I do hope this OP doesn't get misinterpreted to sound like a call to arms. It's likely that person would get a fair amount of shooting in before anyone could react, but I'd say yes, in the end, he'd get shot.
Quite a 'hypothetical', tecoyah, but I'll play along... hypothetically. So, let's say some ANTIFA character barges into an NRA convention, as you suggest, carring an AR-15, and kills "a bunch of people". Well, I sincerely doubt that he'd even be able to accomplish that because he'd be shot dead on the spot as soon as he uncased his rifle, and that's if he were even able to get inside the convention building with it. 'Hypotheticals' are kind of fun, though... how would it be if an asteroid the size of Washington D. C. landed, by chance, right on top of Washington D. C. ? The best thing about being human is our imaginations....
Firearms are not disallowed at NRA meetings and conventions by the NRA. When VP Pence spoke at the convention the secret service demanded no firearms at the convention during his appearance. Now if the venue, city or state has a policy or law stating no firearms in a facility of size or type the NRA's position is to comply with said law or policy.
Who would shoot a murdering ANTIFA member? Oh, again, 'hypothetically', I can imagine any one of millions of stalwart, conscientious American citizens.... Imagination is a wonderful thing, nicht war?
Aaah... do you imagine for one minute that in 2019 ANYONE has a 'convention' with no security? Especially an organization like the NRA which has many deadly, radical America-hating enemies now? My guess, again, 'hypothetically', is that the floor of an NRA convention is probably the safest place on Earth for any law-abiding American citizen to be....
The bottom line is at the event at Indiana Convention Center and Lucas Oil Stadium the membership could carry their weapons if they those to because of the venue's gun policies and local laws. So any murdering ANTIFA member wouldn't stand a chance but, you never know they might be stupid enough to try. Now if the NRA ever held an event is Detroit say Ford Field where guns are banned period except law enforcement, this murdering ANTIFA member might knock a few members off before being shot. The NRA position is to follow the law of the land, they are kind of anal about that. They will argue vigorlessly to change the law, but they do not promote breaking it.
Since it could happen anywhere, there wouldn't be much of a lesson learned from that aspect. What would be extremely clear afterwards, is that having a room full of armed people reduces bigly the number of people hurt by a crazed gunman as being shot to death by dozens of people in a few seconds leaves little time to commit murder.
Please get your facts straight! It's embarrassing. Even Snopes knows you're wrong: Did the NRA Ban Guns at Their Own Convention? A popular claim about the NRA confused that organization's rules with those of the venues at which they held events. Rating: FALSE https://www.snopes.com/fact-check/nra-convention-gun-ban/
I went with...."other... I'll explain" because....... Hitler, Stalin, Mao and many other modern dictators who organized a genocide took away guns from the average person first...... before setting their plans in motion. A mass shooting of NRA members would not change that fact or the understanding of that fact of history by the majority of NRA members and supporters. As you can guess... i was tempted by this response as well...... "Nothing....people would die though."
Vegas shooting was a country music festival. Mostly the same folks you'd find at an NRA convention. That didn't change a lot of minds on the issue, I don't see why anything would be different with OP.
Do you know if the festival allowed concealed carry or bringing in guns at all? Odds are they didn't, so that's why a lot of minds weren't changed.
Just about everyone is armed at NRA conventions; the murderer's demise would be imminent. Far far fewer innocent lives would be lost or wounds sustained than if it were to occur in a GFZ. Having been to a few of these conventions I can attest that the only time the attendees are required to be unarmed (firearms checked in temporarily) is when the POTUS is in the building. Secret Service are the only armed individuals in that situation.