Top villainess manga, stop. Really interesting both in the art department and the plot, the characters and world design. It greatly explores the actual consecuences that the usual Heroine-Villainess relationship and development would have in both the psyche of the villainess, the heroine and all those around them. The ML is not exactly a yandere, but he's not merciful either. The iconic relationship between the Hero, the Villain and the Monster are alluded time and time again, each time exposing it from the perspective of a different character, and the protagonist, the antagonist and the monster have a very good number of chapters to discuss thingsamongst them.

Really a gust of fresh air in a genre populated by clones differenciated by very small details. If I had to recommend something similar, It'd be "The Person in a Villainess". Very different in plot but also astounding in both art and worldbuilding.

TLDR: Top Villainess manga. Very few works of this genre reach the heights this one does.