What do you think they should have done? Confiscated his weapons?
A simple 5150 72hr hold and mental evaluation, with his death threats = a RSO (restraining order and confiscation of firearms) . He shouldn't have been in school, should have been tried, shouldn't have been able to get firearms and shouldn't have been able to keep them.
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But raising the age to 21 will stop this in the future? Sure....