Who would even consider locking people up before they commit a crime? If we are going to consider locking up mentally ill people before they hurt anyone, then we should also lock up all the other people who may be dangerous.
Over 30,000 people die in the U.S. each year in car accidents, so we should lock up anyone who owns a car. Drivers are inherently scofflaws. They drive over the speed limit. They make rolling stops. They ignore pedestrians. A truly rational person could not own a car. People who own cars are obviously dangerously demented. Lock them all up.
Firearm-related deaths are catching up to the slaughter on the highways. Someone who is paranoid enough to think they need a gun for some reason and deluded enough to think that a gun could keep them safe is surely dangerously crazy. Lock them all up.