In The Grifters, why was Lilly acting more like a grifter than a mom to Roy?

Lilly is not able to act like a real mother to Roy because the grifting way of life infiltrates every part of her being.

Lilly shows how grifting is not just a job as much as it is a way of life. Lilly has had to grift throughout her life. Her life has been filled with competition and conning to get what she needs or wants. From her economically challenged background to giving birth to Roy at such a young age, Lilly believes that living is one "long con." She shows this in how she competes for food with Roy when he is young and in the way she cultivates the grifting tendencies within him. Lilly is unable to show any capacity for nurturing and selfless love because the life she leads is one where "the grift" never ends.

Thompson shows that being a successful grifter means using people as a means to an end. It requires a hardness of heart. This defines Lilly because she uses people in order to get what she needs or desires. This is her way of life. She views using people, including Roy, as a path to her own happiness. She ends up taking his money because she needs it. She kills him as a way to achieve it. Lilly sees motherhood as a part of the grift. Thompson seems to be suggesting that when we embrace the grifting life, a mode of living that views people as a means to an end, it precludes all else. Nothing else matters but "conning the mark." As a result, Lilly cannot behave as a mother. She can only be a grifter.