Actually No Yes I Did That Was Completely Intentional - Tumblr Posts
I am just as glad as any other Kuroshitsuji fan that Season 2 isn't canon, because the ending is pretty fucked up. Especially the more you think about it.
Like, it brings up soooo many questions; can Sebastian not make any more contracts or feed on any other souls if he's eternally serving his now-demon master? Cause if not, then... he's gonna starve—
And even if he were allowed to feed on uncontracted souls to stay sane (more on that later) it's still quite an unfortunate way to go. Contracted souls are considered "gourmet" compared to uncontracted souls. The more emotionally troubled a person is, the more "seasoned" their soul is. When a demon is in a contract, they're able to "season" a soul to their likening until the contract is completed and the meal is fully prepared, so to speak.
Eating only uncontracted souls would basically be the equivalent of consuming only bread and water for the rest of your life.
The worst part? (Incoming headcanon dump btw) demons most likely don't get that sweet release of being able to starve to death like humans do.
Mostly because it's impossible for a demon to die in general unless they're killed by a grim reaper. But if that's the case, why do they even need to be consuming souls in the first place, one may wonder? Same reasons minus "or else they'll starve to death" basically. Or rather, more specifically, to maintain their mental state.
If a demon goes too long without feeding on a single soul (however long that really takes, given that we see Sebastian fast for three years and he's perfectly fine) they would start to gradually lose their sanity and by extension, their "humanity," to the point where they can't think straight. To the point where they can't think about anything other than the fact that they're starving.
Letting their demon side take full control and becoming more and more animalistic to the point where they can't think, period.
We can only pray that Season 2!Sebastian was eventually put out of his misery by the reaper's scythe.
Or y'know have Grell slice demon!Ciel up like a fish. That works, too.