Well, we know that Taim gave them the order to kill Rand in Far Madding. Thus, I don't see why he should doubt that they could achieve it (he just couldn't predict Fain's involvement).

Slayer thinking in WH that no male channeler except the male Forsaken knows how to reach him speaks against the idea that Taim could contact him, because it seems more likely that Slayer truly knows who can find him and who not. The disguise suggests that someone didn't want Moridin or Rand to find out who gave the order, likely a second chance Forsaken like Osan'gar who already clashed with Rand and wants to prevent directly fighting a channeler with access to the Choedan Kal.






The idea that RJ invented a bad assassin for the Shadow, who is obviously still held in high regard of the Shadow bosses (even the DO uses him as an assassin), is simply ridiculous. If Slayer fails to kill someone, it is because the author decides that the victim is further needed in the plot; but not to suggest that Slayer is a bad assassin. In the same way, the Forsaken or the gholam failing to kill the ta'veren or the Wondergirls, shall not suggest that they are completely incompetent.

Many top channelers point out that they can naturally be killed by anyone, if they are surprised and the attacker acts too quickly. For example, even Min manages to wound Semirhage twice with her knife, or Fain to stab Rand. Besides that, we don't know what other powers Slayer has. According to BS there was a deleted scene in TSR, which had told more about his powers. Maybe Slayer can even temporarily stun a channeler. That he casually offers his patron to try killing Rand again in WH, even if it's not clear if Rand is in Far Madding anymore, suggests that Slayer actually isn't afraid of top channelers.