Third Source : Harry Potter and the Deathly Hallows by J.K. Rowling

Source Three  : The Harry Potter Series

"'Don't look so shocked, Severus! How many men and women have you watched die?'

'Lately, only those whom I could not save. You have used me.'

'Meaning?'

'I have spied for you, and lied for you, put myself in mortal danger four you.  Everything was supposed to keep Lily Potter's son safe and now you tell me you have been raising him like a pig for slaughter?-'

'But this is touching, Severus,' said Dumbledore seriously. 'Have you grown to care for the boy, after all?'

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...Throughout the entire series, Snape has been dishonest with Lord Voldemort. He has convinced him that he is indeed on his side, and is working as a spy for Dumbledore.  His true loyalties, however, lie with Dumbledore and the Order of the Phoenix. His job is to give certain pieces of information to Voldemort, just enough so that he still thinks him a credible source, but not enough that their plans are given away. It is a very difficult and dangerous job and only one like Snape, with an incredible flair for dishonesty can pull it off. So here we would have a classic double agent scenario. Snape is deliberately being ‘kind’ to Voldemort so as to remain in his deepest confidences, but there is the fact that he is not really on his side under it all. This is, I think, a magnificent example of “excessive courtesy must contain deceit.” Snape is as courteous to Voldemort as possible, does his bidding, and finds information for him, and all the while is disloyal and, in reality, should not be trusted by Voldemort. In this series, deception definitely moves the plot forward in many ways. And so it is, I believe, with many book series’...