Part of it was that even though we had followed the villainess's point of view periodically for much of the book, we got only her narrow planning for each stage of the plot, and then, at the end, it was revealed it was not only an objective of pure selfishness, it was incredibly petty and small.
Not in accord with the scale of the evil she had done to commit it -- which can done, but not as a surprise twist. No, the immense littleness of the objective versus the vastness of what, and who, was sacrificed to obtain it should have been clear all along to pull that off. It made her look blind to what she was meddling with, which really doesn't work as a surprise.
Also not in accord with the way she had thought of it -- vaguely, but as a grand and epic thing. If she had been aiming for something more transcendental but thoroughly evil, that could have worked. It's hard to surprise readers and have it work when you have shown a point of view and omitted significant details, but if the point-of-view character seems -- disturbed, it can be rendered plausible. For her aim to be merely shallow and selfish did not have the level of disturbed thinking that works.
I did reflect on whether it was just that we got shown her thoughts falsely. If a more strange and grandiose scheme would also have come off badly. It is possible, but I think the pettiness was what really sealed the problem.