Slayer migt have used a quicker and deadlier weapon than a knife, e.g. a crossbow with poisoned bolts, besides that, Slayer has Powers we don't know about yet, according to BS. If Asmo considered for a moment (namely when he said "You?" ) that Slayer might have come to rescue him, this could have been the crucial eyeblink he acted too slow. We know that Slayer took the Gray Man's crossbow in TDR and likes to use deadly poison. But as I also mentioned several times now, if - as RJ stated - Fain with his dagger had killed Asmo in this situation, the professional assassin Slayer certainly could have succeeded as well.