Here is a good article on the two shootings.
The real reason conservatives should be outraged that police killed a white youth - The Week
No one knows how many black kids are shot by the cops each year because no one knows how many people cops kill each year. Cop shooting deaths are self-reported to the FBI and there is no standard for reporting them and no central reporting authority for tracking improper killing by cops. By the way, there are at least 400 people shot by cops per year according to the self-reported statistics. About 100-150 (with a high of about 200) cops are killed in the line of duty per year - half or more are typically killed in car accidents. I would be interested in the statistics of civilians killed by cops in car crashes, but strangely enough there's no central database for that statistic either.
The thing that is clear is that cops are trained to aggressively kill others to prevent even the possibility of danger to themselves and that they are willing to tolerate considerable civilian casualties to prevent even a single cop death.