The correct answer is Arial Sobel.
There's a pepper shaker in front of the victim's plate, but no salt shaker. In fact, there's no salt shaker anywhere on the otherwise properly set table.
Arial Sobel and Marcus Seaver were having an affair, until Glenda found out and sent her husband a note. The Seaver money was all hers and she planned to leave Marcus with nothing unless he cut off Arial. Marcus conceded and abruptly told Arial it was over. Arial didn't like this.
Arial found rat poison in a shed and dumped it into the salt shaker. Marcus was complaining about the tasteless food and she was counting on him to be the first at the table to use salt. After his death, she would take the shaker, clean it and refill it with salt. And if someone else happened to grab it first? Well, she hated Glenda, too, and wasn't too fond of her own Gil.