The trouble with something like 'kindness' is how do you take something that can have a meaning so grey and abstract - that has been proven time and again that it can be superficial and utilised as a means of manipulation and selfish gain - and apply it reliably to any action to call it 'care'. Especially when the person concerned isn't even human.
We don’t know to what level demons can connect with or express emotions as understood by humans. What we do know about Sebastian, though, is that he has grown to respect and admire Ciel, and regards his soul as unique. We know that he has proven himself capable of 'care' in the sense that he will act for his master’s benefit even outside of the bounds of what his contract requires. He wants Ciel's soul for his own, and he wants it at its very best, but that in itself has led him to give a level of attention to his master that surpasses all basic expectations for their covenant.
Sebastian has previously mused that devils do not possess complex malicious intent, so even if his interest can be put down to a pure want for Ciel's soul as his dinner, it would not necessarily make his actions 'evil'; it may be selfish, but there is no spite in what he does. More than that, Ciel already anticipates that Sebastian has no true feelings and has been intentionally trying to keep him at arms length in case of just such an event despite his need for comfort, continuing to maintain the shield even as they continue to grow closer, always reasoning that he allows Sebastian as close as he does for the pure fact that he can be controlled.
And to that end, Ciel will continue to receive Sebastian's loyalty and companionship until the completion if their deal, regardless of the motivation behind it. When the time comes, he will die knowing that he got precisely what he asked for, and that the care and attention he has received from the demon - even if it was false - was no less than what he paid for. He will go with no regrets as to that.
And in this story where the reality of one's own world is governed by one's individual perception, does it really matter in the end whether or not Ciel truly was in control of Sebastian's 'loyalty' towards him, so long as he believed it enough to derive comfort from his presence? Does it matter if Sebastian's 'care' was no more than contractual obligation, so long as Ciel received it? Will it make the ending any less what Ciel wanted if Sebastian shows that he cannot continue to change or 'grow' from the experience after the fact?
Regardless of intentions - or of a demon's capacity for human-type emotional growth - we can see and understand through his actions that he considers Ciel's soul to be worth something that at least looks like kindness - and from an apparently otherwise callous and apathetic creature, maybe that can be considered its own type of caring.