A explicação de Snape para Bellatrix:
"I should remind you that when Potter first arrived at Hogwarts there were still many stories circulating about him, rumors that he himself was a great Dark wizard, which was how he had survived the Dark Lord’s attack. Indeed, many of the Dark Lord’s old followers thought Potter might be a standard around which we could all rally once more. I was curious, I admit it, and not at all inclined to murder him the moment he set foot in the castle."
(Half-Blood Prince, Chapter 2, Spinner's End)
"I have done my utmost to have him thrown out of Hogwarts, where I believe he scarcely belongs, but kill him, or allow him to be killed in front of me? I would have been a fool to risk it with Dumbledore close at hand.”
(Half-Blood Prince, Chapter 2, Spinner's End)