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In order for such a situation to flow smoothly, a degree of trust between the policeman and the approached is required. 70 years ago that would have been possible in America, but now it is a low-trust society. That police unit was likely used to facing genuinely dangerous individuals and acted in an accordingly paranoid manner. Such is life in a low-trust society: well-meaning people are prevented from being helpful and innocents get harmed.
Even then, a sane and professional cop could perhaps have overcome pressure, interpreted the situation and realized he wasn't facing a threatening individual. But this man's unstable mind tipped the scales and made it a tragedy.