Just wait. Next we'll learn that he was deaf, or autistic or something which inhibited him from responding to shouts coming from some distance away.
What I still want to know is, if someone is 'gesticulating' with a gun but no shots have been fired at anyone yet, how does that give the police the right to steam in and shoot first? (See NJ's hypothetical as well - '[getting] retaliation in first')
Reports said he was 'frightening' passers by with the gun. Not necessarily walking up to them, threatening them with it (for gain), just that they'd seen him and were frightened by his behaviour.
Fair enough, you can't approach such a person without some expectation of being fired upon, so action was required.
Why not try to get the armed person to fire one shot into the ground, to demonstrate it is real, before shooting them?