What is the theme in Mourning Becomes Electra by Eugene O'Neill?

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.