It is not inherently "reasonable" to perceive "a threat regardless of whether one exists." An irrational / paranoid subject may easily perceive a threat where none exists. The standard of reasonableness must be applied to the notion of perception, not the notion of self-defense or the notion of "criminal justice."

But there's a further complication. In the event that it is not reasonable to infer that there is a threat, a subject may still claim that such an inference was reasonable, given extenuating circumstances. Then it becomes a matter of he said-he said, with practically no check on the assertion of the policeman. Should someone brandishing a realistic looking toy gun, like 12-year old Tamir Rice, be shot dead on sight? If the cop in question wouldn't have shot a 12-year old White kid on sight, then he might claim some sort of "reason" to assume that a Black youth was more likely to have a real gun than a White youth would be.

It would seem that Black people in America have target painted on their backs.

Again, there must be room to challenge the subject's belief in the primacy / validity of the extenuating circumstances. There must be an examination of potential racist motives when an officer uses deadly force (whether this racism is conscious or unconscious).

Otherwise, if we flip the situation, practically all Black people pulled over by an armed White policeman have reason to believe that they should (at the very least) flee in order to protect themselves.

For starters, we could develop a test that would (with reasonable certainty) reveal racism in applicants to the police force. Anyone of any race who believes in the inherent inferiority / suspectability of anyone because of race should not be hired.