“Shouldn’t police know there was a threat before killing someone?”
No, that isn’t the standard for police, or for any one else for that matter.
Even private citizens, who have a more stringent standard than police, are allowed to shoot to kill if they have a reasonable fear that their life may be in danger. So if someone kicks your door in at night, you don’t have to wait to see a gun to shoot him.
But in this case there was no kicking in the door at night. No reduced visibility. No sign of a weapon. No threatening words or gestures. So where was the threat?
And Supreme Court decisions have said police can shoot an unarmed suspect only if there is a threat to the officers or the public. Again, where was the threat here?