I genuinely don't think we do, or at least we didn't until very recently. I believe that when the headline would read something akin to "Police officer shoots man" the police would automatically be given the benefit of the doubt, that we would inherently assume a reasonable explanation. I'm not really arguing as to whether or not that is reasonable, I'm merely remarking on the contradiction of how we react when reading such headlines.We do.
That's why the headlines are "Cop kills unarmed black man"
Not "Man kills unarmed black man"
The uniform is implied by "cop".
And when they're out of uniform they're referred to as "plainclothes" or "undercover".
Whether or not you're supposed to be "scared and angry" or whether you see it as an unfortunate consequence of granting authority to people to commit violence, is up to you.
What if the headline instead read "Pizzaboy shoots man", or what about "teacher shoots man". Of course there is the possible consequence of police perhaps having to kill in their profession, but the proposed headlines aren't actually mentioning any such thing.