I'm not the greatest thinker in the world so the easiest answers are the ones i like. First off lets rule out the forsaken,they knew he was teaching rand but besides that they didnt know where he was he could have taught at a different location and if he did stay there,they would not be able to find him without asking around and if they had wouldnt that be suspicous?just a thought. Next rule out the wise ones because they seemed completly clueless to the fact of who he was and thinking about it under ji e toh something would constrain them to not be able to without letting the cara carn know .whew tired yet.Also the people who dont show up until later books.as far as taim? maybe but i think rj really meant that it was a character we have seen throughout the book and taim wasn't there. The simple answer i like is this. It was Fain.I know the wording his scream still hung in the air,but we have only seen what the dagger can do if it cuts,imagine it through the heart! As for motive no real true answer.He didnt like the forsaken ,he sure didnt work with them as demonstrated with his conversation with Alvarian. Numerous times asmodean could have seen fain at darkfriend meetings or at Shayul Ghaul itself,he probably knows what happened to fain there and at Sadar Logoth.as for just being there? well we see at the tower before he leaves he takes a couple choice darkfriends and says hes going after rand and he does have that connection and knows where rand is all the time.Somehow he figured out asmodean was teaching him and thought to get rid of him because he thought to be in danger of not killing rand himself.We all know that revenge is the only thing that drives fain know so its a perfect motive.Whew i'm exhausted.well gotta go:! "Al Shaidar a Machin Sin"