Harry Potter and the Deathly Hallows II

'I thought... you were going... to keep her... safe...'
'She and James put their faith in the wrong person,' said Dumbledore. 'Rather than you, Severus. Weren't you hoping that Lord Voldemort would spare her?'
Snape's breathing was shallow.
'Her boy survives,' said Dumbledore. 'Her son lives. He has her eyes, precisely her eyes. You remember the shape and colour of Lily Evan's eyes, I am sure?'
'DON'T!' bellowed Snape. 'Gone... Dead..."
'Is this remorse, Severus?'
'I wish... I wish I were dead...'
'And what use would that be to anyone?' said Dumbledore coldly. 'If you loved Lily Evans, if you truly loved her, then your way forward is clear.'
Snape seemed to peer through a haze of pain, and Dumbledore's words appeared to take a long time to reach him.
'What - what do you mean?'
'You know how and why she died. Make sure it was not in vain. Help me protect Lily's son.'
'He does not need protection. The Dark Lord has gone-'
'- the Dark Lord will return, and Harry Potter will be in terrible danger when he does.'
There was a long pause, and slowly Snape regained control of himself, mastered his own breathing. At last he said, 'Very well. Very well. But never - never tell, Dumbledore! This must be between us! Swear it! I cannot bear... especially Potter's son... I want your word!'
'My word, Severus, that I shall never reveal the best of you?' Dumbledore sighed, looking down into Snape's ferocious, anguished face. 'If you insist...'