
i think she said something about her real father being the enemy family of her current family? since her family now has the power to destroy ie. poison and stuff , her real father's family have the power to heal. thats why she can do the antidote and thats why she will be kill. shes the enemy's daughter.
well this is if i remember correctly

Still, slicing off her neck is just nonsensical. If she were indeed from the enemy family, keeping her captive, taking advantage of her would be the best solution, no?
Also I just use this story as one example of many nonsensical lore in isekai stories because I notice similar pattern. Why did this thing happen in original story? Well, because things happen for the sole purpose of convenience.
What I don't get about these stories is the nonsensical lore. Okay, you're illegitimate daughter, you're outcasted, fair. But now you're exposed that you're not even their real daughter by blood (you have different dad and mom), and the duke just straight out cut off your neck? No banishment, not expelling, not disowning, not even returning her to her original family, just straight up murdering her?? Logic doko??
But I mean I like Ian enough to keep me coming back to this story.