How did IT die?

A Wrinkle in Time is the first in a series of five books by Madeleine L'Engle, referred to as the "Time Quintet."
In the book, Meg Murry, her younger brother Charles Wallace, and her classmate Calvin go on a journey to save Mr. Murry, a brilliant scientist and father of the Murry children. He is on another planet called Camazotz, where a large, evil, disembodied brain referred to as IT is holding him captive and controlling the minds of everyone on the planet. Charles Wallace at first tries to defeat IT with his extraordinary intelligence, but IT takes him instead, controlling his body and mind. Meg eventually realizes that hatred and anger make IT stronger and that only love will help her save her brother. She uses her love for Charles Wallace to set him free from IT's control. I do not believe IT actually dies in the book.