And yet the idiom always refers to human rottenness. Shakespeare, for instance, seems to have worked the idiom into Hamlet as both a motif representing human sin and a principle of story structure. The inciting incident of the play—Claudius’s murder of his brother King Hamlet—creates an odor of moral rot perceptible to watchman Marcellus in Act I when he famously states, “Something is rotten in the state of Denmark.” The odor turns Hamlet into a literal ratcatcher.
In Act III scene ii, Hamlet forms a plan to catch his uncle like a rodent by staging a play that will prick his uncle’s conscience. He nicknames the play “The Mousetrap.” Immediately afterward in scene iv, Hamlet discovers someone hiding behind the arras in his mother’s room and, thinking it’s Claudius, Hamlet cries out, “How now? A rat?” and stabs the intruder, who turns out to be the busybody Polonius, not Claudius. Hamlet becomes a murderer of the innocent just like Claudius, and catches himself in his own guilty mousetrap. Shakespeare would like us all to know: Hamlet is the rat. We are all the rat.
