"The Fastest Gun In The South" line raises the hairs on my arms every time.
I think three of the answers are practical;
1) "The Fastest Gun In The South" - 1) A PERFECT title for the film sequel if it ever came to be;
2) It is summation of the man Django became-
a person with the sand and ability to fight Slavery's injustice.
Obviously the intention of this line at the end-placed out of chronology- sums up Django's memory of Dr. Schultz--the refinement of Django's gift, the mutual respect between the two, but most importantly---King sees a legend in the making-- A man of a suffering people with ability to become a hero for all time by freeing his people.
2) The fastest gun in the West could not exist until the movement West. Django is set a few decades before that culture came to be---the moniker "The Fastest Gun In The South" suggests before the white man tamed the West by a mix of genocide, lies, and genuine hard work--then created the white hat hero Cowboy image to erase the real history--
The fact that a black man rose up to rescue many from one of the worst evils man has ever unleashed---legal slavery---with the tools one associates with the cowboy myth---is a noble, clever, and downright socially revelant reinvention of the Cowboy .