One horror story is about a grandparent who constantly criticized their grandchild's appearance. They would make mean remarks like 'You're so fat, no one will ever like you.' This really damaged the child's self - esteem.
A bad grandparent horror story could be where the grandparent favored one grandchild over the others blatantly. They would give expensive gifts to only one grandchild while ignoring the rest. This created a lot of jealousy and resentment among the grandchildren. The unfavored ones felt unloved and left out, and it affected their relationships not only with the grandparent but also among themselves as they grew up.