He who sups with the devil should have a long spoon (слэш)

Severus watched him leave with an expression of forced satisfaction, eight torturous years spent largely under the same roof and he was finally leaving for good, it should feel like closure, he should feel relief. But what he does feel as he watches the back of Potter's head descend down the front steps and into the carriage, feels a lot closer to regret, and as his stomach writhes unpleasantly like a nest of angry vipers, he watches him walk away, walk away forever.