Some bullies simply enjoy wielding power. That's especially so for strong kids with aggressive reaction patterns and a lack of empathy. As many know firsthand, there are a large number of grandparents raising their grandchildren today. The most common reasons their adult-children have abandoned their children are either from circumstances surrounding divorce or from drug or alcohol abuse. In the loving interest of grandchildren, many grandparents step in, taking full responsibility for their grandchildren's health and well-being.
Any alternative is unthinkable.