Quote 1
"Hide them all, then," he croaked. "Keep her – them – safe. Please."
"And what will you give me in return, Severus?"
"In – in return?" Snape gaped at Dumbledore, and Harry expected him to protest, but after a long moment he said, "Anything." (33.129-130)
Here, we see the moment of Snape's shift from evil to good – his betrayal of Lord Voldemort because of Lily's endangerment demonstrates the power of his love over his desire to serve the Dark Lord.
Quote 2
"Karkaroff intends to flee if the Mark burns."
"Does he?" said Dumbledore softly… "And are you tempted to join him?"
"No," said Snape. […] "I am not such a coward."
"No," agreed Dumbledore. "You are a braver man by far than Igor Karkaroff. You know, I sometimes think we Sort too soon…" (33.144)
Dumbledore's comment that Hogwarts Sorts its students too soon implies that Snape's bravery should perhaps have placed him in Gryffindor – his true identity, which nobody else knows about, shows him to be as courageous and loyal as any true Gryffindor.
Quote 3
"So the boy… the boy must die?" asked Snape quite calmly.
"And Voldemort himself must do it, Severus. That is essential."
Another long silence. Then Snape said, "I thought… all these years… that we were protecting him for her. For Lily."
"We have protected him because it has been essential to teach him, to raise him, to let him try his strength," said Dumbledore, his eyes still tight shut. "Meanwhile, the connection between them grows ever stronger, a parasitic growth: Sometimes I have thought he suspects it himself. If I know him, he will have arranged matters so that when he sets out to meet his death, it will truly mean the end of Voldemort."
Dumbledore opened his eyes. Snape looked horrified.
"You have kept him alive so that he can die at the right moment?... You have used me… I have spied for you and lied for you, put myself in mortal danger for you. Everything was supposed to keep Lily Potter's son safe. Now you tell me you have been raising him like a pig for slaughter…" (33.182-185)
This is a kind of double betrayal on Dumbledore's part – of Snape, and of Harry himself. We feel cheated, just as Snape does – how could Dumbledore manipulate them (and us) like that?