Stopping there in his slow drift backwards, Snape allows a smirk to spread across his lips.

"I don't need revenge on you, anymore." Well, that's not the most warm and cuddly of cryptic statements ever, there, Severus. "Having you remain in my debt is revenge enough."

...something says that's not entirely what he means, but Merlin forbid to let Snape say anything straight unless he must.