Who was responsible for mutilating pets and chickens in Maycomb?

That would be Crazy Addie.

“Once the town was terrorized by a series of morbid nocturnal events: people's chickens and household pets mutilated; although the culprit was Crazy Addie, who eventually drowned himself in Barker's Eddy, people still looked at the Radley Place, unwilling to discard their initial suspicions”