IMO, this makes it also rather unlikely that any other servants, who we didn't even see, were connected to the murder, e.g. that Graendal was just Compelling servants in the Palace, when Asmo bumped into her.

Also Sammael can't be the killer, because RJ still said after his Sammael's toast comment, that - if it is revealed in the books - he will probably reveal the murder in the killer's PoV, which isn't possible for Sammael (at least if we don't jump back in the timeline just for the revelation, as others also believe might happen regarding Graendal...).