The question is if there were no more hostages, why shoot him?
Because the cops probably didn’t know that ALL the hostages were out and there was no guarantee that he was the lone perpetrator nor that he wasn’t carrying a bomb.
It’s a battle zone and most times facts don’t come out until you have perfect hindsight.
Now HAD he come out, arms raised and surrendering and they shot him then there’d be a good point for malfeasance.
But he didn’t.
The question is if there were no more hostages, why shoot him?
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The question is why he wasn’t shot sooner.