In another memory, Snape ministers to Dumbledore after Dumbledore rashly puts on the ring of Marvolo Gaunt and suffers a blackened and burned hand because of the ring’s curse. Snape’s potions buy Dumbledore a year of life, but nothing they do can prevent the curse from killing him eventually. Dumbledore makes Snape promise to protect the students at Hogwarts if Voldemort gains control of the school, to help Draco stay out of trouble as he tries to carry out Voldemort’s instructions to kill Dumbledore, and to kill Dumbledore himself when the right time comes.
Later, Dumbledore tells Snape that after Snape kills Dumbledore, there may come a time when Voldemort seems to fear for the life of his snake (which will mean that the other Horcruxes are destroyed or threatened). At that moment, Snape should tell Harry the truth: that when Voldemort sent his killing curse at Lily Potter and blasted his own soul to bits, a piece of Voldemort’s soul bound itself to Harry’s. This event is the reason that Harry can read Voldemort’s mind and can speak Parseltongue. As long as Harry lives, so will Voldemort. The only way Voldemort will die is if all of the Horcruxes are destroyed and Voldemort himself kills Harry, who is in fact the seventh Horcrux. Snape is furious, accusing Dumbledore of raising Harry as a lamb for slaughter and using Snape by falsely telling him he was protecting Lily Potter’s son.
Later, after Dumbledore’s death, the portrait of Dumbledore tells Snape to give Voldemort the correct date of Harry’s departure from the Dursleys and to plant the idea of using decoys, so Harry can escape. In the pursuit of Harry, Snape burns off George Weasley’s ear accidentally, while trying to protect Lupin from a Death Eater.
Snape goes to Sirius’s house and steals the small fragment at the end of the letter Harry found, simply because the page is signed “Lots of love, Lily.” He tears Lily’s picture out of the photograph of Lily, James, and Harry.
Finally, the portrait of Phineas Nigellus tells Snape that Harry and Hermione are hiding in the Forest of Dean, Hermione having mentioned the place while opening her magic bag. Dumbledore urges Snape to take the Sword of Gryffindor to them under the right conditions, and Snape leaves, on his way to put the sword under ice and use his own Patronus (the silver doe) to guide Harry.
The memories over, Harry wakes up in the headmaster’s office.
Snape has no chance to fight or stand up to Voldemort, and thus has no time to demonstrate his true heroic character before he dies. His final actions are as tightly cloaked in mystery as everything he has done throughout the series. Sadly, his death accomplishes nothing, as Voldemort is simply pursuing one more misguided and doomed scheme to acquire the power to beat Harry. Fortunately for Harry, Snape has time for one last act, extracting his memories for viewing in the Pensieve, the headmaster’s privilege (since the Pensieve is in the headmaster’s office).
The viewing of Snape’s life story in the Pensieve is very satisfying, as it explains everything mysterious or contradictory that we have witnessed about Snape throughout the series. His connection with Slytherin and past as a Death Eater are all real, but his animosity toward Harry was never malice, but simply irritation at seeing Harry’s father reflected in him. We knew since Harry Potter and the Prisoner of Azkaban that Harry’s father had persecuted Snape, but we never saw that Snape’s hatred of James Potter was counterbalanced by a much stronger emotion. Much to our surprise, everything that Snape has ever done throughout the series has been motivated by love. The sudden revelation that things were not as they seemed, and that something else was going on all along that we are only now aware of, is called irony, and it is one of the most pleasurable experiences an author can produce. When the events in question concern a character who has fascinated us and held our attention through seven books, the experience for the reader is rare and special.