They should be better than the average person at *doing their jobs*! We don't accept "they're only human" for other professionals who make egregious mistakes that lead to death - doctors who make serious errors, child care workers who leave a child in a hot car or let one wander off through an unlatched gate. Hell, even bartenders are held responsible for exercising good judgment in not over-serving. Police are (theoretically, anyway) trained in threat assessment and deescalation and procedures intended to keep everyone safe, and when officers forget or ignore that training they need to be held accountable. Because despite the fear-mongering used to justify the ever-increasing militarization of police, our law enforcement officers aren't actually under siege on the American streets.
Racism isn't always as explicit as seeing a black face and perceiving it as threatening. It can also operate on a subconscious level - heightened awareness and exaggerated threat response in a black neighborhood because of a "gut feeling" that the neighborhood and situation are inherently more dangerous than the same type of call in a white neighborhood, for example. Most of the LEO in my community are retirees from larger departments and I've heard them talk about the neighborhoods they wouldn't walk into without a gun drawn and finger on the trigger (which I'm sure is hyperbole to some degree, but does reflect the mindset). I'm sure none of them think of themselves as racist and would argue that it is the socio-economic status and crime statistics that made them fearful in those neighborhoods, but the end result is still the same - police walking around afraid, and therefore jumpy/quick to react, in black neighborhoods in a way that they aren't in whiter parts of the city.