After the many harsh punishments(Banishment, torture, etc.) of the innocent ponies of zephyr heights, Prince Legacy is forced to do the unthinkable. His mother cannot be reasoned with and he can't stand by and watch the suffering continue. In Cozy's twisted mind, she believes that her son did this for power and that makes her feel proud of him before dying.

Throughout history, Cozy Glow is referred to as the mad queen. Similar to how Queen Mary 1st(Bloody Mary) is seen in our world. Thankfully, her descendants turned out much better.