There are several reasons that Elizabeth feels, and accurately so, that Abby wants to kill her. One reason that Abby wants to cause harm to Elizabeth is spite. Elizabeth, once she suspected the affair between Abby and John, fires her and kicks her out of the house. This was damaging to Abby's reputation; when a girl is fired, the word spreads through town, and rumors started. Because of those rumors, Abby couldn't find employment anywhere...