Seeing it’s the third prince and not the second prince on the cover, I assume it was the second prince who poisoned Eve
1. Because he was working with that lil b*tch ‘friend’ all along to try and get rid of her so she won’t be able to inherit her father’s Dukedom and gain those riches for themselves which will give them more powerful positions
Or
2. Because, perhaps, Eve did not have all that weight as she currently does now when he proposed to her before. Eve may have gone through something that led her to gain that weight within these past 5 years she was engaged to him. The 2nd Prince didn’t like that so he’s desperately trying to ‘break it off’ (by killing her ig lol) without losing the support of the Duke who has a considerable amount of influence and could help him become the future king.
And that’s my inference based on the cover and these past 3 chapters hehe.