That is, what eats away at the murderer is a false belief that becomes a "rats on a wheel" obsession in the brain. The consuming behavior is what Sayers points to as the cause, not to any actual cheating.
The "trap" of jealousy is detailed in a book otherwise devoted to train tables: Five Red Herrings. In the book, Farren's wife is entirely virtuous as she "comforts" the eventual victim, Campbell. She is an upstanding woman who treats her husband's fears about Campbell as a lack of trust--on his part.
Her husband is not a fool. She appears to have "given" herself emotionally to another man. Yet she clings to her self-assigned role of virtuous and compassionate woman who is above it all.
When she argues with Wimsey that couples should have full trust between each other, he agrees. He then tells her, in a quite friendly way, that if he were her husband he would know that she wasn't cheating. It would offend her opinion of herself, the role she has adopted as long-suffering, tolerant wife. When Mrs. Farren bridles at Wimsey's "insults," he points out that her husband's assumptions were less offensive (to her) than Wimsey's.
Farren, in fact, went walkabout. When Wimsey and the police finally track him down, he doesn't appear to fully understand what he is running from, what accounts for his feelings of relief. Wimsey is sorry to have to send him home to a supernally patient and forbearing wife who will smother him in her own perfections. Poor Farren might end up committing murder eventually.
The pain and rage of jealousy appears to be a state of mind that Sayers understood well, in terms of its destructiveness to a human's sense of self and sense of integrity. She delineates it with analytical honesty in her novels.
