Here's how it happens:
* Bellatrix is fighting against Harry Potter, Ginny Weasley, and Luna Lovegood. She's a skilled and powerful witch, and she manages to disarm Ginny.
* Molly Weasley steps in to protect Ginny, and the two witches duel. This is a crucial moment in the battle, and the tension is palpable.
* Bellatrix taunts Molly about her children's safety. This is a deliberate attempt to provoke Molly and get her to lose control.
* In a fit of rage and grief, Molly casts a powerful spell that hits Bellatrix directly in the chest. Bellatrix falls to the ground, dead.
This moment is significant because it shows Molly's fierce maternal love and the lengths she'll go to protect her family. It also marks the end of Bellatrix's reign of terror and brings justice to the many victims she tortured.