Racism was something imported to the New World from Europe in the 1700s.
First the European white man killed the red man. Then in Mexico, Central America and South America the Spaniard killed most of the native peoples. Then all the white men imported black slaves -- this was a Spanish, Dutch and English pan-European operation.
Slaves-Sugar-Rum.
Those cops died in Dallas for reason of European evils committed centuries ago.
This has nothing to do with the petty politics of hate we have today -- history comes back to haunt us.