[In fairness to Sebastian, if he'd known this would involve being slammed by the feelings one witnesses rather than just being privy to memories, he would perhaps have exercised a little more caution ("perhaps" and "a little" being the keywords).
As it is, he's completely unprepared for the onslaught of emotion. Not only is it in such sharp contrast to the possessive anger he was feeling; it's also completely unlike anything he's ever felt before. Disappointment, frustration... those are familiar. But sadness? If he's ever felt anything of the sort it's been a vague, watered-down version. Certainly never years' worth of it, from multiple relationships and encounters, all condensed into one moment. It's enough to have him take a stumbling step back, gripping the feather so tightly he'd crush it if it didn't dissolve a moment later.]
Grief...? [Is that what this is? No, it's-- failing someone important?]
I have not-- [He has not failed Ciel (has he?). A different man might weep with the intensity of the emotion, but it's too alien a reaction for Sebastian. He does remain standing where he is, knuckles white around his now empty hand, overwhelmed if only for a few moments.]