Apparently foresight was 20/20 here as well. California ruled that this guy was too mentally unstable to possess a firearm. It turns out, their ruling was quite prescient.
"Do we start taking away rights because you think some one "may" commit a crime? "
No. But we do take away people's rights because they can't discern fantasy from reality. There's nothing noble about presuming someone's Constitutionally protected right to make their own decisions, if they don't have the mental capacity to make those decisions, to include owning firearms.
"How do you propose we determine who is and who is not "mentally fit"
Many states (to include California) have procedures in place that strip citizens of their rights - (ie institutionalization). These procedures protect, respect and affirm the due process rights of the citizen.
"...There's nothing noble about preserving someone's Constitutionally..."