It fits in with every character who doesn't attack people directly, but rather tries to undermine them using plots. IMO, Asmodean's death excludes all the male Forsaken, Lanfear and Semirhage - who all use direct methods to kill their foes. Graendal, Moghedien and Mesaana are sneaky.

I think Graendal was sneaking around the palace starting plots all over the place. She ran into Asmodean and got a huge fright. She would have considered him to be on Rand's side and at his full power, and she probably thought she'd walked into a serious duel for her life. I'm quite sure she wove balefire with all her might, threw it at him, and flung herself through a gateway. Asmodean would have done the same, had he been unshielded.