Aku no Hana, or "The Flowers of Evil", is one of the longer manga series I've read through the years, but I finished it within 3 days - it was that captivating. The story is quite mature and the physical abuse the FL inflicts on the ML might turn you off since there's no "revenge plot" or "getting her back" for what she does as a manipulative, abusive, and blackmailing girl wrapping the ML around her finger to do her bidding.
Aku no Hana is a master at visual story-telling. There's not as much actual reading to do on your part, and some of the subtle writing/artist decisions might not connect with you until the second reading, but those more psychologically aware will catch on to many of the emotional cues of the characters or when certain panels pay specific attention to an object, an expression, or something that would otherwise seem benign or unimportant.
I was recommended this when in pursuit of a romance manga. While I was actually in anticipation of a romance, I was greeted with something far different, but I stayed for the ride all the way through regardless. There's not much in the way of romance, so if you're eyeing that "Romance" genre tag in anticipation of getting your fix, drop that expectation now. Don't get me wrong, there IS romance included in this manga, but it's not the focus and
it's not between the two main characters. The ML does hook up with someone after the time skip but the relationship is more so used as a plot device to wrap up loose ends between the ML and FL and have the ML reach some needed closure and help him find answers and fill this void in him.
I don't think it's a spoiler for me to mention that there is a time skip about half way through. Some don't like that, but I didn't mind and in fact it renewed interest in me since the danger the ML and FL were getting themselves into and the chemistry that progressed between them kept building up to the point that I was unsure where the author was going to end them at, but the time skip solved that dilemma and relieved the reader of that tension of "what could possibly come of this when the the ML has nothing left to lose now and he's cornered himself into obeying the FL's every whim?" During the final volumes, I was questioning whether or not the new female character introduced would actually be the FL years later but didn't recognize the ML or forgot about him. To the author's credit, I wasn't convinced with certainty if she was or not until the reveal finally came and we saw the FL post-time skip near the ending chapters. Getting to finally see her again and wondering how she progressed as a character after years of being away from the ML was cathartic and kept me antsy to see
how much more "tame" she actually got and reading the conceding words "normal man" uttered by the FL after how much she pushed the ML into further social deviancy to satisfy her perception that he was as much of a freak and outcast as she was.
All in all, Aku no Hana - "The Flowers of Evil" - kept my eyes glued to the pages and while it's not the type of book I could pick up and read through multiple times, it's not one easily forgotten, either.