I certainly don't claim to have proved that no American police officer had ever been racist. That would be preposterous, given the history of real racism in the US. But in the cognitive sense, racism is only an overgeneralization, a stereotype that discriminates against certain individuals. We all resort to stereotypes, especially when having to make snap decisions.
The question is whether racism explains the aggregate behaviour that shows up in the overall numbers of police killings of suspects. What's the best explanation of the data? Racism doesn't cut it because there's a rival explanation that does better. The economic class-based explanation explains the shootings of poor blacks and whites, whereas the race-based explanation has to ignore all of those police shootings of poor whites, just as the corporate media do for their cynical storyline.