While I like the premise, the MC seems to soft kind boring? With this sort of premise, It'd be better to make the MC so obsessed with her revenge that she couldn't let anyone else in the heart, no matter how nice and loving her new family was. I feel like because she has other things she cares for beside her revenge, it makes her seem weak and not fully devoted to the reason she SOLD her SOUL for!
Basically, to me at least, she seems to lack true resolve. I'd prefer her single-mindedly focused on her revenge, truly closed off to others, and dealing with the fact that aside from her revenge she has no future now that she has sold her soul. That would be more tragic and befitting of the premise and the route the author seems to want to take but isn't fully wiling to commit to.
Of course, this is my opinion as it would suit my task. I'm sure with the pretty art, Kuroshitsuji vibes, and being a villainess story; there's a audience for this that probably completely disagree with me.