I'm really loving this premise so far, especially the dynamic in which Roselia interacts with the duke and Ain.
Not sure why I feel this way, but I hope Roselia resolves this fear of the duke and his bloody activities at some point. I feel like Roselia earnestly learning about how Ain has long since started killing and growing to be a 'villain' would create a more honest relationship between the main characters. Of course, I wouldn't be surprised if that never happens and the series devolves into fluff, which is fine in its own right (and I'd still read it), but I think it would be more interesting if Roselia comes to terms with her character as part of a family of 'villains' and doesn't shield her eyes from the violence.
Beyond that, I think an interesting, albeit slightly outlandish possibility based on how the story is going, is that Ain somehow comes into danger, say an assassin or some other person makes an attempt on his life, and Roselia is the only person available to make any attempts at saving him. For example, the duke and Ain are being held captive, and an enemy of their house comes to know that the relationship between Roselia and the duke is beyond normal and tries to win her over and betray the family perhaps based on the fact that she's not a true 'villain' of the family (or some other bizarre reasoning,; afterall, we don't know enough about the story yet). At this point, I'd hope Roselia realizes that she values her family more than abstaining from bloody violence and comes to terms with everything, perhaps even personally trying to free the duke and Ain even if it means killing the captors herself...
Of course, that's all just conjecture. Regardless, thanks for scanlating the series so fast. I'm eager to read more!