Alicia’s treatment of other people - often for her own selfish reasons - highlights her cruel and heartless nature.
Despite being her son, even Alec cannot feel any warmth for such a cold and distant woman.
In the opening paragraphs of the novel - before we have even met her character - he says that “My heart doesn’t bleed for her.”
Mr. Cave, the piano teacher, is speechless at her beauty, “Such a beautiful woman, God love her. So …”. Yet soon after he says this, she cruelly dismisses him because of her disgust at his poverty.
In this way her exterior beauty is juxtaposed with the ugliness of her personality, emphasising her spiteful nature.
She heartlessly employs emotional blackmail to get her son to go to war. She sees it as a personal victory when he finally decides to go - with the description of her eyes as “triumphant blue”.
The writer shows her as not at all worried for his life, but instead enjoying the sympathy she’ll receive from having a son at war and the fact that it will hurt her husband.
She cruelly uses Alec as a pawn in her bitter fight with Frederick.