Most crimes occur within the same racial demographics. 90-something % of murders of black Americans are committed by other black Americans; 80-something % of murders of white Americans are committed by other white Americans.
What racists try to imply is that black Americans have no right to complain about a "few" wrongful killings by police until "they" clean up "their" crime problem.
There are so many racist assumptions and slanders bound up in that thinking that it's hard to untangle them all. One is that all black people everywhere are the same "group," so it is somehow the black people killing other black people who are also being killed by police, who are also the same people protesting.
Somehow, apparently, Tamir Rice should have been in the streets cleaning up crime in his community instead of, you know, being 12 and playing with a toy before being executed by twitchy police.
Hard to apply given none of the publicized killings involve anyone accused of murder, but racists aren't know for razor-sharp critical thinking. The other big one, of course, is just to imply that the imaginary one group of black people in the world are simply hyper-violent, and perhaps deserve to be profiled and treated as especially dangerous.
It's so stupid it would be funny, if it weren't so malicious and sick.