And you're right, the first death was the only one where the perp was completely obvious.
So we convict Rahvin. Case closed!
Seriously, though... my memory is fuzzy, but doesn't he yell "Do not abandon me!" when Rand barges in on him and his toadies? Rand thinks he's talking to his sycophant lords and ladies, but I think we were supposed to have gathered that he's talking to another Forsaken in the room. I get where Jordan was coming from, is what I'm saying. We were all supposed to realize that Graendal is noping the heck out of there while Rahvin is demanding she stay and follow what's left of the original plan. Didn't Sammael just spring the trap they were talking about in their big meeting - Mat is supposed to be dead thanks to his darkfriend girlfriend, and Rand is supposed to be coming to Sammael for revenge. Meanwhile, Rahvin, Greandal, and Lanfear (MIA) would be waiting (together, right?) for Sammael to call for help.