While Rebecca describes the struggle between good and evil, My Cousin Rachel explores the nature of good and evil. The narrator, Philip Ashley, shifts from one opinion to another as he desperately tries to discover whether Rachel is a murderer and greedy conniver or a hapless victim of circumstance and misguided men. Because Philip ultimately solves the problem of Rachel by killing her, readers must also judge whether Philip has wrongly killed an innocent woman, or struck a necessary blow against evil. As the novel vividly shows, the presence of evil is not easily recognizable, and the true worth of a person is highly subjective.

My Cousin Rachel also addresses the role economic independence plays in a person's life. Rachel marries Ambrose because she wants economic security. Their marriage appears to threaten Philip's financial security because he has been Ambrose's heir, but Ambrose dies before he can change his will. Rachel, thus, is once again financially vulnerable. After Philip falls in love with Rachel, he wants to grant her the economic security she desperately craves, but Ambrose's will does not permit him access to his estate until his next birthday. On his twenty-fifth birthday, Philip turns over his estate to Rachel with the provision that, if she remarries, the estate reverts to him. What Philip does not realize is that she then no longer needs him. In fact, if she marries Philip, she loses the estate. Philip argues that he would share with her all that he has, but she can attain economic independence only if she remains unmarried. Through Philip and Rachel du Maurier explores the relationship between self-determination and financial independence. Near the end of the novel, Philip, young and desperately in love, is willing to give up both; Rachel, who has known the vulnerability a poor and older woman faces, is not.