Snape Balances his love and respect for Lord Voldemort and Faithfulness to Dumbledore Very Beauifully.He used his known dark magic to cure Dumbledores curded hand.Snape's memories then reveal that Dumbledore had been afflicted by a powerful curse cast on the Gaunt ring, one of Voldemort's Horcruxes, prior to the start of Harry's sixth year at Hogwarts. Although Snape's knowledge of the Dark Arts enabled him to slow the spread of the curse, the curse would have ultimately killed Dumbledore within a year. Dumbledore, aware that Voldemort had ordered Draco Malfoy to kill him, asked Snape to kill him instead as a way of sparing the boy's soul and of preventing his otherwise slow, painful death. Although Snape was reluctant, even asking about the impact of such an action on his own soul, Dumbledore implied that this kind of mercy-killing or killing "by request" would not damage a man's soul in the same way murder would. Snape agreed to do as the Headmaster requested. 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.

He helps Draco Malfoy in turn Narcissa Malfoy By getting into the unbreakale vow with Narcissa. By killing dumbledore he fulfills the wish of Lord Voldemort Dumbledore and Narcissa.