This is based on opinion. Please don't list it on a work's trope example list.
Tearjerker / Where the Red Fern Grows
The infamous ending. If any of you had ever had your dog die, it's made even worse by the fact that they died protecting their master likely from the same fate that they suffered. Old Dan anyway. Little Ann, however, dies of heart break after her brother had died. This book is scarring!
Animal-lovers may agree with Billy's father about the cruelty of the nail-trap Billy uses to catch his first raccoon. The image of the dead raccoon's little fist still clutching its shiny treasure is heart-wrenching.