It wasn't confirmed in ToM that Graendal killed Asmo, just that she was involved with it (like I argued). Graendal also didn't directly kill Mesaana or Aran'gar. It might have been Luc who did it, instead of Isam, who served Graendal in ToM. Even if Graendal considers Slayer to stand below her, it doesn't mean that she wouldn't use him as a tool to kill another one - even a Forsaken - especially since other Forsaken also used him to try to assassinate Rand.

I will buy it, though, if BS said that this was the whole revelation of the murder.. (if this were the case, it would be most pitiful, IMO)