RJ has never implied that there are clues in the first four books as to who killed Asmodean. Why on earth would there be? Asmodean was only captured in book 4. Is Graendal supposed to be thinking about killing Asmodean in book 1 while Rand is drinking from the Eye of the World? That's crazy stuff.

Book 5 established four Forsaken of principle importance (apart from Moghedien who had an altercation with Nynaeve). They were Lanfear, Rahvin, Sammael and Graendal. Lanfear and Rahvin died, and that left Sammael and Graendal as the principle murderers. RJ then provides several conversations where they discuss Asmodean. That's real evidence. What's Slayer got... the statement that he's the Shadow's assassin? Does that mean he killed Adeleas and Tylin too? C'mon.