I see a lot of readers are disappointed with the BE. But, as the translator said at the end of the last chapter, the story really couldn't have ended on a happy note.
The ML is someone who will have 'eternal life' whilst the FL is just a human martial practitioner. Even without the poison in her body, she would have ended up dying before him.
The ML had the option of using his jade bed (the one that made him a gyonshi) to make her an undead as well, then live together forever. But he has lived for 1000 (one thousand) years already and he knows the difficulties, sorrows and loneliness that comes with living far too long. He wouldn't wish that on his beloved. You might say they wouldn't feel loneliness as they would be together, but is it really that easy to live a life where only the two of you remain the same and everything else changes around you? I really don't think so. Even if they go travelling around the whole world, it would still get tiring at some point. And why should he let the woman he loves endure all these just so he can selfishly have someone accompany him?
As for the ML (person with eternal life) falling in love with the FL (person with limited lifespan), I don't think it was something he could have avoided. After interacting with FL for so long, living together day to day, her being the only person he can interact with in this long life of his, it really is very difficult not to start falling in love. He seemed to have been a sentimental man to begin with (talking to trees, reading poetry etc). And even if he did fall in love with someone who will leave his life soon, it is better to have this time of happiness, than to deny himself and act aloof. At least he has the memory of their time spent together and sometimes these memories are what keeps people holding on.
I give my full marks to the translator for doing an awesome job. And I also love and agree to their ending remark. Thank you so much for your work.
Spoiler