Calling backup in the form of a uniform and a black-and-white would be the intelligent thing to do. An honest armed citizen will NOT see that as a lethal threat. Quite the contrary, in fact.
The cop acted stupidly and recklessly, putting himself in a situation wherein a reasonable person would see him as a deadly threat.
Then he killed a citizen.
What do we call that, under the law?
Negligent homicide?
I think the word you’re looking for is ‘murder’.