Professor Albus Dumbledore: We both know Lord Voldemort has ordered the Malfoy boy to murder me. But should he fail, I should presume the Dark Lord will turn to you. You must be the one to kill me, Severus. It is the only way. Only then will the Dark Lord trust you completely. [Flashback of Snape killing Dumbledore]
Professor Albus Dumbledore: There will come a time when Harry Potter must be told something. But you must wait until Voldemort is at his most vulnerable.
Professor Severus Snape: Must be told what? [Shot of Snape going through the halls of the Potter house]
Professor Albus Dumbledore: On the night Lord Voldemort went to Godric's Hollow to kill Harry, and Lily Potter cast herself between them, the curse rebounded. When that happened, a part of Voldemort's soul latched itself onto the only living thing it could find. Harry himself. There's a reason Harry can speak with snakes. There's a reason he can look into Lord Voldemort's mind. A part of Voldemort lives inside him.
Professor Severus Snape: So when the time comes... the boy must die?
Professor Albus Dumbledore: Yes. Yes... he must die. [Shot of Snape collapsing at the sight of Lily's dead body]
Professor Severus Snape: You've kept him alive so that he can die at the proper moment. You've been raising him like a pig for slaughter!
Professor Albus Dumbledore: Don't tell me now that you've grown to care for the boy. [Snape casts a Patronus in the shape of a doe, just like Lily's- the same one that helped Harry find the Sword of Gryffindor in Part 1]
Professor Albus Dumbledore: Lily... after all this time? [Shot of Snape holding Lily's body in his arms while crying]
Professor Severus Snape: Always.