I should imagine that few of The Word Den's readers are actual pigeons (pigeons' reading skills are notoriously poor, though on the positive side pigeons yield to no one in their talent spotting small boats on the ocean), so we are left with the other two possibilities.
Being a police informer is morally rather difficult: basically, do two wrongs make a right? Is this sort of stool pigeon a double-dyed villain who can't even be loyal to his own disreputable comrades? Or is he a brave under-cover agent trying to save lives from the forces of darkness?
Luckily I'm far too cowardly for this to be a matter of urgent practical difficulty.
Fortunately the USA sort of stool pigeon presents no moral difficulties at all. He will be working for a criminal, so he is almost certainly both wicked and doomed.
And serves him jolly well right, too.
Thing Probably Not To Be Today: a stool pigeon. The bird meaning is the oldest, appearing in the early 1800s. The other meanings soon followed, though at first the luring-people meaning didn't necessarily imply any criminal intent. The derivation of the stool bit of stool-pigeon no one's quite sure about, but pigeon comes from the Old French pijon, young dove, from the Latin pīpīre, to chirp.