I kinda think that there was a "rhyme" to what QT did with the script structure here, and a certain service to Django as his lead character.
Simply put, when Schultz shoots Candie, and is in turn killed by Butch...the two main white characters are out of the film(with Schultz being dispatched by yet another white character.)
Now, Django is set on course for a final confrontation with Stephen...the black hero will kill the black villain(and, unlike as with Waltz, will not die in the process.) Indeed, before their final fatal confrontation, we get the scene where Django is hanging upside down and Stephen is revealing to him the full evil depravity of his sell-out "house slave" to the white slave masters.
Schulz was Django's mentor for the first 2/3 of the film, teaching him how to shoot, how to be a bounty hunter, how to assert his authority AS a bounty hunter(Django uses Schultz's verbal techniques and wanted posters to convince the Aussie mining men to give him a GUN, for God's sake!), and even how to dress better. With Waltz now dead, Django has to take over the movie and assert himself as the hero -- starting with that big ol' gunbattle that we just KNOW Django has been aching for against all these white bigots. And look how great Django dresses at the end, after that initial silly blue outfit he wears at Big Daddy's ranch.
By the way, in an interview, Will Smith said one reason he turned down Django Unchained is that HE wanted to be the one who killed Calvin Candie, not Schultz. That's movie superstar thinking -- "its all about me." QT protected his structure(the two white men die and set up the confrontation between the two black men) and let Smith walk.