The question is, if a persons history shows he is so dangerous he should never be allowed to have the means of self-defense, why is he not still locked up? It’s not like a career criminal or homicidal maniac has no other way to harm people than a legally owned gun.
Anyone can buy a weapon privately anyway. You don’t have to go to the gun shop and submit to a background check.
Unfortunately, it is hard to tell what a person’s future behavior will be, unless they are either clinically criminal, or have committed so many repeat offenses that they can be called a career criminal. Which is why many states have “three strikes” laws.
A good illustration of this is found in homicide. The police have long known that homicide can be subdivided between “one shot” domestic disputes, and more typical crime. The one shot murderers almost never re-offend, so after they have completed their sentence they are low risk.