Revenge
Revenge serves as a primary motivation for the play's actions. Seeking to revenge the death of his mother, Marie Brantome, Adam hopes to destroy the Mannon family, especially Ezra.

The Mannon family is a complex web of revenge scenarios: Christine wants revenge on her husband for her unhappy marriage; Lavinia wants revenge on her mother for killing her father; Orin wants revenge on Brant for sleeping with his mother.

Paradise
Paradise is an obsession for many of the play's characters. As a seafaring family, early generations of Mannons had sailed to beautiful South Pacific isles. Orin wants to run away with his mother Christine—an attempt to escape societal norms so that he can sleep with his mother. Christine wants to go with her lover, Adam.

Eventually, Orin does eventually go to the islands with his sister Lavinia. During their visit, she has sex with one of the islanders. In O'Neill's play, the island paradise—offering erotic possibilities and freedom from materialism—becomes a symbol of all that New England society is not.

Incest
Incest and incestuous desire lie behind most of the relationships central to Mourning Becomes Electra. Ezra's daughter Lavinia loves her father; Christine's son Orin loves his mother, and Lavinia and Orin love each other.

While O'Neill presents these relationships as unconsummated desires, Orin does urge Lavinia to sleep with him in act three of The Haunted, hoping that by committing incest that they will be bound together in sin and guilt. His sister refuses.

Sin and Guilt
O'Neill's work illustrates his fascination with sin, guilt, punishment, and redemption. In Mourning Becomes Electra, the sins include murder (Christine's killing of her husband; Lavinia and Orin's killing of Adam); adultery (Christine's with Brant); suicide (Christine's and Orin's); and premarital sex (Lavinia's with the islander).

In a sense, Ezra murders Brant's mother by refusing the sick woman money for food and medicine. Also, Lavinia "kills" Christine and Orin by driving them both to commit suicide.

Orin's feelings of guilt lead him to write his confession, which he threatens to give to Peter if Lavinia marries him. At the play's end, Lavinia's guilt forces her to give up hopes of happiness and to punish herself, as the last Mannon, by rejecting love and shutting herself in the house.