If Graendal died - and IMO there are no convincing reasons to believe otherwise - she wasn't Asmo's killer.

But the simply reason is then, that Graendal certainly won't have another PoV, and it makes not much sense to let another character than the killer reveal who murdered Asmo. Also RJ said in an interview that, if he will reveal it in the books, the killer will probably think about killing Asmo in the own PoV. This simply makes the most sense to create a powerful revelation scene to the most discussed mystery in the whole series. I see no reason why BS should do it in the less probable way. Since Graendal had the chance to reveal it in her last PoV in TGS (and in her several poV before..), but didn't do it, it follows that she most likely wasn't the killer (of course, the Graendaldunnit theory was anyway always very tenuous, if you really look into the mystery of Asmo's death).

Graendal might have sent Slayer, though (she or Lanfear, most likely).