Is Sebastian sad when Ciel becomes a demon?

Maybe yes maybe no. He could be sad because he was looking フォワード, 前進, 楽しみにして to eating his delicious soul. または he could be happy because he can stay with his Bo-Chan forever .

Perhaps "sad" as Sebastian accepts the fate dealt to the two of them, Sebastian & Ciel will be forever together, locked together to walk thru eternity in the Underworld. Not what Sebastian originally contracted and imagined for their final fate.

I think he seemed もっと見る resigned and dissapointed than sad. He definitely didn't seem his normal self. Obviously, the pursuit of devouring Ciels soul has been his primary focus up until this point so being locked into service with Ciel forever with no gain at the end would piss him off slightly I'd imagine. Although, he does seem to accept it somewhat. I guess there's no point fighting what can't be changed.