time until darkness
Sebastian once read an excellent and highly instructive set of letters from a colleague, who commented that: "An ever increasing craving for an ever diminishing pleasure is the formula. It is more certain; and it's better style. To get the man's soul and give him nothing in return - that is what really gladdens Our Father's heart."
This troubles him a little. He is aware that in his dealings with Ciel Phantomhive, he could shave the bargain a little more closely than he is doing. "Fulfill all my desires" - come now, the trope's as old as Faust and Marlowe, and he knows where that's supposed to go.
He should be feeding the boy up with luxuries and vanities, stuffing his immortal soul with lies and sins, prompting him to betray his duties, his neighbours, his servants, and his love. Or he should be sickening the boy with all the things of this world and the next, just to speed up that moment when the boy will say, "Enough," and his soul will be Sebastian's. It is, after all, his job, and he is a hell of a butler.
But the boy's like a cat, and Sebastian likes cats. It amuses him to give the boy wholesome food for body and soul, to keep him alive and desiring for a little longer - a day, a week, a month, a year - and to delay the final repayment that he will eventually request and require.
After all, once the boy is dead and his soul is paid, everything will be different. He won't be Sebastian's own any more. He will belong to Hell, one more soul to feed the constant howl of hunger that rises up from the Depths, one more eternal cry of agony and knowledge.
Sebastian chooses not to think about what he is doing, when he provokes, protects, and promises: he is merely performing his duty in giving the boy what he desires, and keeping the boy for himself, for a little bit longer. This pact of mutual ownership will continue as long as the boy can desire, however much he must suffer for it, and however much pain is caused by Sebastian shielding him.
It's only time. Sebastian has nothing but time. Sebastian intends to enjoy every last moment of his time here with his owner. Besides, it is a butler's job to look after his master, whether or not the master wants it, and whether or not the master may dream of peace and endings and boats going down into darkness among the shadowy cypresses.
And Sebastian is a hell of a butler.