" Either Snape is indeed vulnerable to the lures of the Dark Arts, so unstable that Dumbledore refuses to count his application, or this is a paper trail that he and Dumbledore have laid as part of a strategy. Snape’s supposed vulnerability to the lure of the Dark Arts is part of their cover story, but Snape has never found that lie easy to swallow, and he certainly cannot bear to choke it out in front of Umbridge and his avidly listening students. It was Dumbledore’s idea for Snape to include this ignoble rumor in his cover persona. Let him be the one to perform it for Umbridge. "

― Lorrie Kim , Snape: A Definitive Reading