[The hand to his temple at least makes Sebastian tear his gaze away from the blood and instead meet Ciel's eyes.]
To be remembered beyond the finality of your own lifespan? How human a desire. [Calm, gentle tone aside, that's something like a joke - he knows it's nothing quite so fundamental and understandable with Ciel, and Sebastian doesn't fault him for it. He knows, were their roles reversed, he'd wish the same lifelong misery on Ciel.]
Should I be so kind, to grant you that?
[His own hand moves from Ciel's chin to lightly brush the bangs from his face. Of course, Sebastian isn't kind, and while he has starkly refused to imagine Ciel's death until today...]
I cannot stand the thought of you dying by any hand but mine. [Even if that other hand is chance, or a whim of nature. In other words... he promises.]