When Harry and Dumbledore enter the Pensieve this week, they learn about Voldemort’s ability to alter the memories of his victims, and, consequently, to frame innocent victims for his crimes. Not only does Voldemort show no mercy for his victims, he also shows no mercy for those he sets up to take the blame for him. Dumbledore is quick to point out that Voldemort has killed his own father to avenge his abandonment of Voldemort’s pregnant mother. Voldemort does not stop to consider the fact that Merope had placed Tom Riddle under a love spell and forced him into a union he never would have considered otherwise. Voldemort is not satisfied with simply killing Tom Riddle, however. He also murders his own grandparents, who had nothing to do with Tom and Merope’s failed marriage. Clearly, Voldemort wants to destroy all traces of the Riddle line so he no longer has to acknowledge his Muggle half. Like Draco Malfoy and many of the Slytherins, Voldemort believes that Pureblood wizards are inherently superior to Muggle-born wizards or half-bloods. In some ways, Voldemort’s killing spree is motivated by self-hatred.
While Harry doesn’t initially understand the significance of Slughorn’s tampered memory, Dumbledore’s insistence that Harry find a way to retrieve it indicates its seriousness. Dumbledore has used Harry to get to Slughorn before. Dumbledore purposefully took Harry with him on his mission to recruit Slughorn to teach at Hogwarts, knowing that Harry’s reputation would be enough to entice Slughorn to return against his better judgment. Obviously, Dumbledore is hoping that Harry’s appeal will also be enough to persuade Slughorn to reveal the true memory, no matter how incriminating it might ultimately prove to be. Again and again, Harry’s desire to be a normal student causes him to underestimate the effect he has on other wizards, who continue to believe that he is their only hope of salvation from Voldemort.