The point of this novel is for the reader to point at the main character and criticize her for her evil behavior. But, Haywood isn't writing a novel to help make everyone feel better about themselves. Rather, the reader should identify with Glicera, because Glicera makes some common mistakes that the reader probably makes in other ways. That is the real purpose of the novel, to remind the reader to treat people well.
Notice how Glicera abuses people in her life. When her father leaves her a disappointing inheritance, she begrudges the unfairness of life—instead of mourning her dead father. When she is tricked into giving herself to a young suitor who promises her love but then abandons her, she takes that as license to hate anyone at all. These are symbolic mistakes that represent bad emotional habits that all people share. People often take out their anger out on each unjustly, and people often feel entitled when they shouldn't.
That entitlement problem is really the issue that defines Glicera as a villain. Because she found something to play the victim about (not having enough money to impress her suitors), she feels entitled to some sort of compensation for all the privilege she feels she deserves but hasn't been granted. So she steals a large sum of money from Melladore, and she ruins a few lives to do it, laughing at them on the way. Her evil aspect stems from her sense of entitlement, which is the effect of her being spoiled and privileged.
