Blacks are pathologically insecure, they know that deep down, they are inferior. Their culture is inferior, their collective history is inferior, their people are inferior. Blacks know that their worse enemy is other blacks (as seen how the various ethnicities treat one another throughout various nations) and therefore they will never be able to build and truly great socitey. They all know this deep down and it upsets them. "Nigger" originates from "negro", Latin for black which was merely used to described black people. However, nigger is deragatory and basically means that "you are inferior for being you and nothing will change that". Couple this with a history of enslavement and failure and you have a recipe for lashing out.

tl;Dr Niggers are insecure and they know that they are failures so calling them niggers confirms what they feel inside deep down.