There are more than 6,000 African-American homicide victims a year. That is a very, very high rate, of about 34. (Those rates are usually given in numbers per 100,000.) The overall rate for the US is about 4 per 100,000. A civilized country like Spain as 0.8 per 100,000. A very violent country like Venezuela has 53; Honduras is worse.
Police killings of African Americans are in the range of about 100 per year. So what percent is that of total homicides. Let's see... [does math in head]: 1% is 60, so 2%, rounding up a bit.
Since we don't want police to kill unarmed civilians at all, even one such person in this category is too much. We are outraged, justifiably. It is an abuse of state power, and that brings greater outrage, because it makes all of us complicit in the crime.
Most child abuse is not by the hand of priests. Yet we are more outraged by their abuse than by almost any other set of child abusers. Why is that? The church is supposed to be better than that. So outrage at those scandals is actually a back-handed form of respect: we respect those institutions implicitly, police or church, and hold them to a much higher standard. Suppose parents habitually dropped their 11-year old boys and girls off at pool halls or strip clubs and other places of ill-repute. Yes, you would pretty much condemn that as bad parenting. We expect the Catholic church to be better than a pool hall. It might actually be safer than the pool hall, but when it isn't our reaction is predictably stronger.
The conservative argument attempts to minimize the importance of police shootings by comparing them to the much vaster numbers of killings, the 98% that is not at the hand of the police. The typical liberal argument carries the false implication that the police are the main killers of black people. Both those narratives are really kind of dumb. We are right to be outraged by any abuse of state power, and it doesn't matter very much that this is only 2%. On the other hand, if our main concern is with the violent loss of human life, then the other 98% is also significant.