Sure you can make some leaps about Slayers job earlier, but we have NO proof about his other kills until his POV in WH, where he doesn't bother mentioning killing a FS ... my number 1 reason for ruling him out. Sure he hates AS, but if he loves to kill powerful people so much it seems wildly out of character for him not to think about killing a Forsaken, surely the most outrageous kill of his career!

Then look at this...

LoC
Threads Woven of Shadow

"You know as much as I do," Graendal said blithely, pausing for a sip from her goblet. "Myself, I think Lews Therin killed them. Oh, don’t grimace at me. Al’Thor, since you insist." The thought did not seem to disturb her, but then, she would never find herself in open conflict with al’Thor. That had never been her way. If al’Thor ever discovered her, she simply would abandon everything and re-establish herself elsewhere – or else surrender before he could strike a blow, then begin convincing him that she was indispensable. "There are rumors out of Cairhien about Lanfear dying at Lews Therin’s hands the same day he killed Rahvin."

Note that Graendal takes a pause before telling Sammael that she thinks Asmo is dead? Note how she then goes on to show him that she's got informants close enough to Rand that she knows Lanfear died at the same time as Rahvin, something only someone close to Rand would have witnessed.

This entire scene practically screams that it was Graendal IMO ... we see her servants and learn that she's out there manipulating all kinds of people using Compulsion, incidentally learning that she is a master of the more subtle forms too. We hear that Sammael doesn't think any of the other FS know where she is (which we know Asmo did). She shows Sammael her Lady Basene disguise which means Asmo could have seen it too as she doesn't seem all that concerned with it.

The list goes on...