Recent protests with regard to the shooting of Trayvon Martin in Florida, Michael Brown in Ferguson, and the choking-to-death of Eric Garner in Staten Island, have made a difference. Those protests, combined with a relatively new technology, the ease of taking videos, have, I think, brought us to a turning point.
I don't know if it's unprecedented for a cop to be charged with murder, as Michael T. Slager, the North Charleston, S.C officer in question, has been for shooting Walter L. Scott, a fleeing, unarmed black man. But I'll bet anything the decision to bring down such a heavy charge has never come so swiftly.
What we're seeing, I submit again, is that technology plus protest plus, not least of all, national focus, has brought us to a turning point with regard to police killings/murders.
They will never go as unchallenged/unquestioned as had been the case, as had been routine, in the past.