What do you get when the hero is clueless, and the villain is the world's best chance at peace and prosperity?
Only Villains Do That is the kind of isekai I remember from back before we called them transfer fantasies. It's not just the story of a guy from Earth who gets yeeted into a fantasy world and has adventures which might involve one or more beautiful women. Most isekai will lean heavily on the Earth origin in order to avoid having to build things from scratch. OVDT is old-school transfer fantasy, where the main character's Earth origin is used to explore deeper into a fantasy culture than the denizens of that would can do themselves, because they're too close to it. This story is filled with action, humor, and thought-provoking moments where you have to really wonder if a particular character -- not just our lead -- is right or wrong about something. You don't have to be a philosopher to enjoy this story, but the story might make you start enjoying a little bit of philosophy.
But it's not just that. Seiji, our main character, is extraordinarily compelling not simply because he's a fish out of water, but rather because he's uncomfortable with even himself. He has cultivated such an all-pervasive sense of misanthropy (at least some of which is a displaced self-hatred) that he automatically thinks the worst of people; and now, thrust into a society that laughs at the petty cares of a fairly comfortable Japanese-American musician, Seiji the cynic has discovered he has a core set of unwavering moral principles that rival any hero. He may be forced to play the role of the dread Dark Lord for the amusement of two bored goddesses, but he's going to do it his way -- and right a lot of wrongs in the process.
As fun as it is to enjoy the magic system, worldbuilding, and theatrical moments of dramatic timing, the real reason why this story is so compelling -- and why I am making my first-ever Royal Road review a five star, despite being so difficult to please -- is this central conflict. Seiji is a personally unlikable character who has been dropped into circumstances that, left to his own devices, would have made him a non-idealistic hero. Instead, he is forced to play the role of a villain in order to save his own life and sanity, and realizes along the way that having this power means he's obligated to do some good with it. He's so cynical that he suspects that the fantasy society was specifically designed to piss him off enough to play his role, but he also knows it doesn't matter why; what matters is that people are in trouble, and he can do something about it. As he says at one point, it's not his fault that the best way to be a Dark Lord is to just introduce this society to common decency.
Seiji therefore has to make very hard choices, far harder than if he were acting on his own as a random isekai protagonist, and is toeing a thin and not-altogether-straight line between (the tropes of) good and evil. In essence, he's shifted from being focused on his own survival to pretending to be evil in order to promote what he knows to be right.
The greatest irony -- one that he seems to suspect, and hate -- is that he's become more of a good person on Ephemera than he ever could have been on Earth. The Overton Window has shifted, and he's now confronted with something that's more than just messed up: he's trapped in something far more evil than he can ever be, and unlike the innocent sap chosen to be the Hero, Seiji has the life experience and half-baked knowledge to do something about it.
In other words, the world of Ephemera and its goddesses have offended Seiji, and he's just as willing to play the role of villain to get back at all three of them as he is willing to be an actual hero, regardless of what he or anyone else calls it. Other characters notice this as well, and he gradually gathers loyal followers precisely because the nominally evil villain is willing to stand up for what's right.
One last note on presentation. This story wins top marks for story, character, and grammar. The only part where it's lacking is in its pacing, which is why I'm giving Style four stars and everything else five. The author didn't pace the first book correctly (and has acknowledged this), which means that while moments of dramatic timing are great the moments of narrative timing are off. Even so, this is the only significant mark against the story, and it's certainly better-paced than many big-name publications I've seen. In fact, the narrative timing is relatively easy to fix in the hands of a good developmental editor (I've worked on far worse myself). Odds are most readers would only notice in a print copy.