An officer should only shoot to end an immediate threat to his own life, or a bystander's life.
when Mr Brooks ran off with that non-lethal weapon in his hand, by definition, he was not a threat to anyone's life.
It is a big stretch to insist that because the police officer had a gun, the suspect might use the taser to take the gun.
But there were two officers there. If the suspect managed to tase one of them, the other one would have been able to prevent the suspect from closing in and taking the gun, by using his own taser, or his own nightstick, or even by drawing his own pistol and shouting "don't move!"
Each of these choices gives the suspect another chance to live, without endangering anyone else's life.

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