What should they do? You would perhaps prefer the cops institute a "pre-crime program?"
Death threats are enough to do an investigation. They wouldn’t have had to look far to see what he was buying or what he was hiding in his apartment.
Too often they dont intervene because they havent been trained how to intervene, or given the authority or the discretion. Then there is the question of WHO. Who made the decision not to act. The doctor, the cop, or someone higher?