I don't see Lando as "kill the black guy" as much as "kill a lead character", to the point originally made.
Well, yes, but you also can't say they cast Dee without any intention of having a "cool black hero" in their movie, now can you? :)
As a side note, Lando is the first (and, I think, only) traitor in the plot... if you count out the elephant from Tattoine. And he redeems himself by helping out the white people... OMG!!!