When I clicked your link, I got a 404 error. Hopefully, the below URL will work better:
This type of incident certainly could be ammo against pro-rights groups, but I think it could be chalked up as a "suicidal nut with a gun" really. Sucidal people cannot be effectively stopped unless their condition is known beforehand. I'm sure there were pre-incident indicators that family and friends could have caught on to, but at that particular moment, there probably couldn't have been anything to prevent this from happening.
Even if firearms were prohibited at the meeting and metal detectors were in use, I still don't think it would have stopped this man from committing suicide. It would have only stopped him from committing suicide in the presence of the city council.