Just because the Forsaken can naturally kill someone doesn't mean that generally a Forsaken wouldn't use an assassin like e.g. Slayer to hunt/kill someone (as seen in the books). Otherwise Slayer's PoV wouldn't make any sense.

And the three Forsaken you mention as likely assassins are ruled out as Asmo's killer (at least if one don't grasp fro straws concerning Lanfear). On the other hand, there is nothing to indicate that your favorite for Asmo's killer, Graendal, who isn't mentioned as a Forsaken assassin, would do it. So, your argumentation is pretty contradictory. It would make at least a bit sense, if you supported Lanfear being the killer.