" If Snape ever feels tempted to protest to these good people that their old faith in him was not misplaced, he can remember what happened when Dumbledore succumbed to the flaw in the plan, the affection that makes it difficult to be ruthless. Dumbledore flinched at telling Harry the truth and Sirius died for it. Snape cannot permit such a lapse when so much is at stake. "

― Lorrie Kim , Snape: A Definitive Reading