First, I agree with the poster below: JKR needed an honest editor who wasn't afraid to rein in an 800lb gorilla who refused to practice good continuity hygine.

Second, with much distance from DH and deep reflection, it is abundantly clear to me that Jo simply did not like that Snape became a runaway fandom favorite, slightly out of her control. His ignoble death on the floor will always represent, to me, Jo flexing her authorial perogative to leave Harry no glory competition at the tale's end.