– Emma Larkin, Finding George Orwell in Burma (2005)
In English, synonyms for informer tend to be vehement rather than metaphorical. A rat is a despised animal but not a betrayer. Comparisons to Judas or Benedict Arnold are usually of the “as bad as” type – name-calling rather than imaginative comparison. Turncoat has the idea of a group member being used against the group – as with the ax handle and the tree – but is not so much a comparison as a label or nickname referencing switched uniforms (see definition of metonymy).
Stool pigeon is the genuine article, comparing the methods of con artists to the methods of hunters. Hunters used to tie a pigeon to a perch or stool to lure other pigeons. Con artists used a fake customer to lure the unwary into rigged games of chance. This meaning is from the early 1800s, according to the OED. By the mid-1800s, stool pigeon also came to mean a police informer, a criminal used to catch other criminals.
Photo: Karl Stull