After all this time?.. - -Always

Severus Snape: It will contain a curse to your hand for the time being. It will spread, Albus.
Albus Dumbledore: How long?
Severus Snape: Maybe a year.
[Snape turns to leave, but Dumbledore stops him]
Albus Dumbledore: Don't ignore me, Severus. We both know Lord Voldemort has ordered the Malfoy boy to murder me. But should he fail, one should presume that the Dark Lord would 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. [We see Dumbledore being killed by Snape, as shown in Half-Blood Prince] There will come a time when Harry Potter must be told something, but you must wait until Lord Voldemort is at his most vulnerable.
Severus Snape: Must be told what?
[As Dumbledore speaks, we see a worried Snape running inside the Potter home]
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 that it could find: Harry himself. There is a reason Harry can speak with snakes. There is a reason he could look into Voldemort's mind. part of Voldemort lives inside him.
Severus Snape: So, when the time comes...the boy must die?
Albus Dumbledore: Yes, yes, he must die.
[We see Snape arrive in Harry's bedroom and collapsing in horror at the sight of Lily's dead body]
Severus Snape: You've kept him so he can die at the proper moment. You've been raising him like a pig for slaughter.
Albus Dumbledore: Don't tell me now that you've grown to care for the boy. [Snape casts a deer Patronus, the same one that helped Harry find the Sword of Gryffindor in Part 1] Lily? [crying Snape is shown holding Lily's body] After all this time?
Severus Snape: Always. So, when the time comes...the boy must die?
Albus Dumbledore: Yes, he must die. And Voldemort himself must do it. That is essential.