Y'all aren't talking about an evolution, it's a variation in how the word is said. People already explained this in the thread.
Trigger vs Trigga
Teacher vs Teacha
Number vs Numba
Etc.
Dropping "er" and replacing it with "a" is standard African American vernacular, n***er is no different aside from the "difference" we assigned it... And even with that, we still cling to the negative connotations and actions associated with it.
How often do black people start a negative comment on our people with "N***as _____ _____ _____"? Now how often do y'all hear POSITIVE comments concerning us start that same way?
When black men and women fight and/or kill each other, are they calling one another n***a out of love??? It's still a dead/hurt black person but it's "different" if they say it with an a, right???