Hetty is a young, uneducated girl, who tries to use her beauty and feminine wiles to get what she wants out of life. Lost in the fantasies and dreams of a little girl who wants to live out a Cinderella story, Hetty makes terrible mistakes, including believing that Arthur will marry her and make her into a great lady.
As part of her determination to remain as socially available and to not ruin her chances to find her prince, Hetty commits murder, she leaves her baby, Arthur's baby, to die on the road. She is guilty of the most horrible crime, abandoning her innocent child out of a desire to return to her former state of being. Hetty lives in a world dominated by denial and fantasy, not reality and responsibility.
In addition, her selfishness is particularly disturbing with regard to how she treats Adam Bede, a decent, hardworking man who really loves her. She uses him, and discards him, wounding his heart without care or concern for his feelings.
Hetty does change at the end of the novel, thanks to the intervention of Dinah, but her crime of murder and her indifference for the feelings of Adam, her materialistic attitude and desire for jewels and fine clothes make her a shallow, selfish girl.