No, it's the concept of "innocent until proven guilty" and even if you'd like to craft a scenario next where we're going to talk about Officer Friendly stopping a rape-in-progress, yes, that person is almost certainly guilty, AND the punishment for that crime is usually not death, AND the cornerstone of our justice system says that the officer in question, no matter how pure of heart he might be, cannot exact a death sentence on a clearly guilty person because that is not how justice works.

A cop killing ANYONE, be they a bystander, or a suspect, or an assailant, should be RARE. It should be notable.