“But he’s a very good Occlumens, isn’t he, sir?” said Harry, whose voice was shaking with the effort of keeping it steady. “And isn’t Voldemort convinced that Snape’s on his side, even now? Professor... how can you be sure Snape’s on our side?”
Se Lorde Voldemort duvidasse de Severus Snape, Snape estaria morto. Todo o conceito de Double agent é muito complicado, é difícil confiar em alguém.
Em segundo lugar, o tópico que estava sendo discutido anteriormente era muito sensível para Harry, já que se referia à morte de seus pais.
“Professor Snape made a terrible —”
“Don’t tell me it was a mistake, sir, he was listening at the door!”
“Please let me finish.” Dumbledore waited until Harry had nodded curtly, then went on. “Professor Snape made a terrible mistake. He was still in Lord Voldemort’s employ on the night he heard the first half of Professor Trelawney’s prophecy. Naturally, he hastened to tell his master what he had heard, for it concerned his master most deeply. But he did not know — he had no possible way of knowing — which boy Voldemort would hunt from then onwards, or that the parents he would destroy in his murderous quest were people that Professor Snape knew, that they were your mother and father —”
Era natural que Harry desconfiasse de Snape, depois que ele percebesse que Snape era em parte responsável pela morte de seus pais.
Atualizando a resposta com base nos esclarecimentos recebidos:
Harry could see him sweating in the torchlight, his white skin contrasting strongly with the black of his hair and beard.
“Snape!” he shouted. “Severus Snape!”
“Snape has been cleared by this council,” said Crouch disdainfully. “He has been vouched for by Albus Dumbledore.”
“No!” shouted Karkaroff, straining at the chains that bound him to the chair. “I assure you! Severus Snape is a Death Eater!”
Dumbledore had gotten to his feet.
“I have given evidence already on this matter,” he said calmly. “Severus Snape was indeed a Death Eater. However, he rejoined our side before Lord Voldemort’s downfall and turned spy for us, at great personal risk. He is now no more a Death Eater than I am.”
Harry Potter and the Goblet of Fire, Chapter 30, The Pensieve
É assim que Harry sabia que Snape era um agente duplo espionando Voldemort.
“Severus,” said Dumbledore, turning to Snape, “you know what I must ask you to do. If you are ready... if you are prepared...”
“I am,” said Snape.
He looked slightly paler than usual, and his cold, black eyes glittered strangely.
Harry Potter and the Goblet of Fire, Chapter 36, The Parting of the Ways
Então Harry sabia, Dumbledore pediu a Snape para fazer algumas tarefas para ele no final de Harry Potter e o Cálice de Fogo.
Existem muitas outras dicas diretas que sugerem que Snape estava em contato com Voldemort. Por exemplo
“Oh yeah,” said Sirius sarcastically. “Listening to Snape’s reports, having to take all his snide hints that he’s out there risking his life while I’m sat on my backside here having a nice comfortable time... asking me how the cleanings going -”
Harry Potter and the Order of the Phoenix, Chapter 5, The Order of the Phoenix
Snape informa Harry sobre Voldemort sabendo sobre a conexão entre Harry e Voldemort.
“It is enough that we know,” said Snape repressively. “The important point is that the Dark Lord is now aware that you are gaining access to his thoughts and feelings. He has also deduced that the process is likely to work in reverse; that is to say, he has realized that he might be able to access your thoughts and feelings in return -”
Harry Potter and the Order of the Phoenix, Chapter 24, Occlumency
Se Harry colocar todas essas coisas juntas, é óbvio que Snape está em contato com Voldemort e também que ele é capaz de acessar informações sobre Voldemort. O que implicaria que Voldemort confia em Snape. E também o fato de que, Voldemort não matou Snape.