"Which was awfully short-sighted of us, wasn't it."

Severus is inexplicable, there; for a moment he seems distracted with his own thoughts, perhaps over the mention of Bellatrix. Perhaps it's just that while he always thought they'd lose - an acceptable risk, for someone who only wanted to practice the Dark Arts and found no point in living otherwise - he couldn't have predicted what kind of staggered, decaying finish they'd endure.