The case Graham v. Connor, 490 U.S. 386 may help to explain this. In a use-of-force case, "courts must identify the specific constitutional right allegedly infringed by the challenged application of force, and then judge the claim by reference to the specific constitutional standard which governs that right", therefore "The notion that all excessive force claims brought under § 1983 are governed by a single generic standard is rejected". Such claims "invok[e] the protections of the Fourth Amendment, which guarantees citizens the right "to be secure in their persons . . . against unreasonable seizures," and must be judged by reference to the Fourth Amendment's "reasonableness" standard".
In invoking the notion of "reasonable", the court is referring to the fact that a person chooses an action being in possession of certain knowledge, and using that knowledge plus reasoning, to judge an outcome. So when a suspect appears to be armed, the officer has to decide whether the weapon is real and whether the suspect is likely to use it against the officer. When one conjectures that a lesser degree of force could have been used because it turns out that the suspected weapon was a plastic toy, one is appealing to knowledge not available to the officer at that time. In Graham, the court held that the legal question
is whether the officers' actions are "objectively reasonable" in light
of the facts and circumstances confronting them, without regard to
their underlying intent or motivation. The "reasonableness" of a
particular use of force must be judged from the perspective of a
reasonable officer on the scene, and its calculus must embody an
allowance for the fact that police officers are often forced to make
split-second decisions about the amount of force necessary in a
In other words, the judgment is made by reference to the objective facts of the circumstance, and not the subjective emotional state of the officer. As the court put it, "The 'reasonableness' of a particular use of force must be judged from the perspective of a reasonable officer on the scene, rather than with the 20/20 vision of hindsight".