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.
Religious people have faith that they somehow have had the truth revealed to them. If you can get them to reveal how they have come to base their eternal happiness on their particular faith, you would probably think they were dangerously sick. Lock them all up.
The same goes for parents, smokers, drinkers, and people who drink soda or eat fried foods. They are all mentally ill. If you do not fit into one of the groups above, then you must be seriously depressed and therefore dangerously mentally ill. Lock everyone up.
Get over it already. Being alive means you are going to die somehow. The world is not perfectly safe and never has been. Without some risk, life would not be worthwhile. If they come after mentally ill people now, who will they come after next?