Quando / Como Snape se tornou um agente duplo? [duplicado]

Snape se torna um agente duplo momentos depois da citação que você descreve.

O contexto é a chave aqui e, pelo som, você está considerando a citação fora de contexto. Snape e Dumbledore não se encontraram acidentalmente - é uma reunião pré-arranjada por Snape.

The corridor dissolved, and the scene took a little longer to reform: Harry seemed to fly through shifting shapes and colors until his surroundings solidi- fied again and he stood on a hilltop, forlorn and cold in the darkness, the wind whistling through the branches of a few leafless trees. The adult Snape was panting, turning on the spot, his wand gripped tightly in his hand, waiting for something or for someone. . . His fear infected Harry too, even though he knew that he could not be harmed, and he looked over his shoulder, wondering what it was that Snape was waiting for—

Then a blinding, jagged jet of white light flew through the air. Harry thought of lightning, but Snape had dropped to his knees and his wand had flown out of his hand.

“Don’t kill me!”

“That was not my intention.”

Any sound of Dumbledore Apparating had been drowned by the sound of the wind in the branches. He stood before Snape with his robes whipping around him, and his face was illuminated from below in the light cast by his wand.

“Well, Severus? What message does Lord Voldemort have for me?”

Dumbledore assume que a reunião foi para entregar uma mensagem de Voldemort, daí a sua desarmação de Snape, e Snapes medo. Eles não são aliados neste momento.

Logo depois disso, Snape explica que ele está lá para avisar Dumbledore que Lily está em perigo, e implora para ele mantê-la segura. Ele até explica que pediu a Voldemort para não matá-la.

“You disgust me,” said Dumbledore, and Harry had never heard so much contempt in his voice. Snape seemed to shrink a little, “You do not care, then, about the deaths of her husband and child? They can die, as long as you have what you want?” Snape said nothing, but merely looked up at Dumbledore.

“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.”

Snape se torna um agente duplo na última palavra.

"Hide them all, then," [Snape] 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."

Isso se transformou em um acordo mais concreto para atuar como um agente duplo na próxima página, na próxima cena em que Dumbledore e Snape falam após a morte de Lily:

So only after Voldemort's "death" did Snape become a double-agent? 
Why was Dumbledore not more surprised to see Snape then? Only because
Dumbledore is not one to show such an emotion?