Voldemort was taunting Harry saying that he chose to let his friends die for him. He gave him one hour to show himself in the Forbidden Forest.
Harry went back to the castle and saw everyone in the Great Hall. Fred, Remus and Tonks are dead. Firenze is injured. Harry drops Snape's memories into the pensieve to watch it. Lily and Severus Snape became friends and he fell in love with his best friend. When they were young, Petunia hated magic and Lily thought it was because of Snape, that he hurt her sister. But even though she got mad, Snape and her were still friends. Then they met James and Sirius. Lily found them disgusting. Then, she became a Gryffindor student.
Snape want to be a death eater, so Lily really got mad. Then Snape requests Albus to protect Lily but they still died in the hands of Voldemort. Snape swore to protect Harry. Albus knew that Snape was Voldemort's spy but now he wanted to protect Hogwarts students. It was Albus who told Snape to kill him in order to save Draco from the burden of murder. Albus also knew about Nagini. Then Albus decided when was the right time to tell Harry that the very last Horcrux is in him (Harry). Thus, he must die in order to destroy the last Horcrux. And Albus used Snape for the greater good. Snape sent the sword and it was his Patronus which was the silver doe.

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My Chapter Summary On Harry Potter and the Deathly Hallows
Fanfictioncredits and honor to Author J.K. Rowling who will always be my inspiration in writing.