I mean i think its called dark path for a reason (instead of for example evil path) and as i mentioned i dont think she will ever go evil for the sake of it. And as were playing the MC ourselfs its unlikely that hes gonna die which would lead to Eliza probably changing a lot.Its a bold decision by the dev to take this path. Eliza is the most popular character in this VN. The dev risks alienating a lot of readers if things go wrong. While I applaud the dev for an intriguing twist, sending Eliza down the dark path would be a step too far for me and I suspect for many readers as well.
And while im not sure if Vampire is a great idea, Eliza needed something to level her up and become (somewhat) equal to the mc otherwise she would always been a liabilty as a battle companion, and theres no way she would have been okay with staying home all the time (and it wouldnt be safe anyway and would suck being departed with her nonstop like in the pirate chapters).
So far i will gladly take Eliza becoming an Vampire and strong herself over being the damsel in distress not being able to travel with the mc together.
And i think most Eliza fans should try to see it like that considering the last pirate chapters without her (which i didnt enjoyed).
Of course there could have been other solutions which would have may been better but were lacking lore about vampirisim in this world (or any other kind of possible ascension) to be really able to judge it.
But yeah, there's a possibility for it to ruin her character, i just hope it wont happen.