This is an excellent question, and it can best be answered by refering to the theme of jealousy, spite and revenge, and how it is shown to pervade all of the characters, no matter what their social class or position. All the characters in the novel are shown to be jealous, spiteful and to take revenge on those who they can. Jealousy and spite is of course experienced by the servants in this novel as they are jealous of the power and wealth that their masters and mistresses possess. However, because of their position and their lack of power, they are only able to take their revenge on those who are actually below them.
You might like to consider Mrs. Wikins, the servant of Allworthy, as a good example. She has to put up with abuse from Miss Bridget because of her position. However, Mrs. Deborah in turn goes on to abuse the townspeople. The servants therefore are shown to emphasise an important--and rather cynical--human truth. Jealousy, spite and revenge are pervasive instincts that lead us to punish those who are below us. They are present in all sectors of human sociey.