Rand was using an angreal to channel and pulling up all sorts of LTT memories for weaves when he confronted her. She had warned Asmodean not to teach him too much or make him too powerful. Her conclusion would have been that Asmodean had lied and gone back on their bargain.

Lanfear may not be the only reasonable candidate, but certainly it's not an "outlandish" theory to suggest she's the killer.

Personally, based on what Larry said I actually think it's a lot more likely that she IS the killer after his statements. I think that Jordan's refusal to say she is Cyndane probably means that there's a HUGE backstory on what happened in Finnland that we can't even guess at fully. Moiraine and Lanfear might be tied into one the same way Slayer is two people at once - and he's gone into that Tower before.