Yes, I want people to have that ability. Then the authorities could take a look at my claim, interview others and see if it holds any weight, and if it does they could take the matter further. And if they (law enforcement, law, state, and mental health professionals) decide I'm full of crap, or that I'm wrong, then great, at least we investigated the issue.
Do you know how many mass murders could have been prevented that way? These things never come out of nowhere. People, often family members and neighbors/acquaintances or coworkers or teachers, can tell when someone is disturbed or a "ticking time bomb." We need to give them a way to report this and we need to give the state power to investigate these claims.
So yes, you bet your *** I'm in favor of allowing citizens to report someone they think is "off."
Most of the time, by the time these types of people "cause a disturbance" it's too late.