Near the end of the novel, Max de Winter confesses to his second, unnamed wife that he killed Rebecca. We only hear Max's side of the story, as told to us by a wife who wants very much to believe his version of events. In this version, Max, though anguished, very much justifies the murder as deserved.
Why was he justified in killing Rebecca? First, he tells the second Mrs. de Winter that Rebecca was unfaithful to him. They had worked out a deal in which she would pretend to be the perfect wife and hostess as long as he did not interfere with her various affairs. However, rumors of her affairs reached Max's ears, indicating that Rebecca was not maintaining her end of the bargain. When Max confronted her at the boathouse where she had been meeting her lovers, he told her he was going to divorce her.
Not about to be bested—and terminally ill with cancer, though Max did not yet know this—Rebecca told him she was having her cousin Jack's child and that Max would have to raise it as his own. Driven to rage, Max shot Rebecca, then took her corpse out on a boat and contrived to make her death look like a suicide.
According the second Mrs. de Winters, Max was manipulated into killing Rebecca by Rebecca, who wanted to die. The second Mrs. de Winters also finds the revelation that Max is a murderer brings the two closer together as she pledges to stand by her man.
The second Mrs. de Winters paints Rebecca as an evil, heartless woman. As we have no other version of the story, hers is the one we tend to believe. However, we might also question a man who felt it was permissible to kill a wife for infidelity.