Simplify the following text for a Grade 4 native English-speaking person.
Original: Snape 's memories also provide Harry with the information he needs to ensure Voldemort 's final defeat , in the form of conversations Snape had with Dumbledore .
Simplified: Snape killed Dumbledore in the sixth book because he has asked him to do that . Voldemort believed Snape killed him because Voldemort had planned Snape to do that .