@AllTheFeels
I have to disagree, she explained very well why she rejected him. "what about after that?" and "I want you to remain as my savior in my memories." suggests that to her, his marriage proposal is no different from the duke's offer to adopt her. She fears that if she accepts it he might turn out to be as abusive as the duke. He's the only person who has been kind to her, she doesn't want anything to sully that image. And she's only known him for like a couple days, which in honesty makes her rejecting him more realistic.
And even if he turns out to not be abusive or shitty in anyway, "I do not wish to live thinking of you as my shackles." pretty much makes it obvious that she doesn't want to live as Karina anymore. If she marries him now as Karina, she'll have to live the rest of her life under that name. Might not seem like a big deal but she'd have to worry about how he react if he ever finds out or if anyone else finds out the real Karina died of the carlet disease (noble families who have an infected fall to ruin). She'll end up dragging him down if that happens and he might end up resenting her.
In the end she stays with her abusive family because she is used to them being shitty and has learned what to do in order to avoid pissing them off. I am on the fence on whether it really was the duke that killed her, he gains nothing from killing her.