And even if he did see a weapon, that isn't a reason to shoot somebody. If you come upon somebody, particularly somebody in their own home, who happens to have a firearm that isn't a reason to shoot them. It may be a tense situation for a police officer, since from the point of view of the homeowner they may not know what is going on, and the police officer has to deal with another armed person, but the police officer should be an expert in that kind of situation.
After all they expect non-police officers to be able to determine when it is, or isn't necessary to use deadly force in far more complex situations.
If an unidentified person starting strolling around my yard in the middle of the night with a flashlight, I’d pick up a gun before going to investigate. Doesn’t mean the cop has any right to kill me.
And “Put your hands up! Bang!” within one second? No one could comply with that order.
This looks like 2nd degree murder to me.