YouTuber “possumpopper89” has crafted a 6-minute video that does a fairly good job of explaining why law enforcement officers (and some “regular Joe” citizens with defensive firearms training) sometimes shoot unarmed people.
People who lack this kind of training (which is probably close to 95% of the general population) hold the mistaken belief that officers wait until they see and identify a weapon in a suspect’s hand before deciding to open fire.
That is simply not the case much of the time. In many instances, it isn’t possible to visually identify a weapon due to lighting conditions or attempts by dangerous criminals to conceal a weapon even as they bring them into play.
The simply fact of the matter is that you can never react as fast as the other person acts. It requires you to observe the other person’s actions, orient (analyze and synthesize the incoming data through the lens past experiences, training, culture, and genetic filters), decide on a response, and then act to carry out that decision.