Since it would feel weird putting my entire post into a spoiler box, I'll just announce the danger in the first line here and be on my merry way. *ahem*
Spoilers for the initial premise and one branch of SMT: Nocturne below.
This is a bit of a borderline case, since the realization of how wrong he is doesn't hit him by the time the ending where he gets his way concludes, but in every other ending it's strongly foreshadowed that he doesn't realize what he's bringing about.
I suppose I should provide some context, first: Shin Megami Tensei nocturne takes place just after the apocalypse. The world has been destroyed, humanity decimated to what can be counted on a single hand, and the earth as we knew it has been replaced with the "Vortex World", an enclosed circle of a place that is home to demons.
What makes this premise unique in nocturne, however, is that this apocalypse was not an act of destruction for its own sake, but a part of how God plans to reincarnate the world after it reaches the limits of potential in its previous existence. Each of the small number of humans who survived the end have earned the right to create a "Reason" within themselves. This basically means they get to come up with a personal philosophy that they would have become the the core of the new world. Once they've established their Reason, demons who would support their philosophy gather beneath them and form their army. The strongest army may climb the tower God has created, and the human who reaches the summit of the tower can have the world reincarnated in the way they desired.
One human established the reason of "Yosuga", a world where strength and beauty are synonymous, and the strong are free to prey upon the weak, allowing the world to advance rapidly thanks to the endless competition of life.
Another human established the reason of "Musubi", a world of solitude where every human lives in a paradise of their own mind's creation, but with the caveat that each individual lives in isolation. The people they would encounter in their personal paradises are merely shadows their minds created, and it is impossible for one person to ever reach another.
For my purposes, I'll be concentrating on the third of the primary reasons to awaken in the game. Now, calling any one of the surviving humans the "villain" is a little unfair, since the player character ally with any of them, or fight all of them. As the player character was given the body of a fiend (the highest class of demon) on the eve of the apocalypse, he was deemed unworthy of creating a Reason of his own, and so enters the Vortex World neutrally. However, as relative as the game's morality is, there's one character who has a stronger right to the title of "villain" than the others.
His name is Hikawa, and he was the man who caused the apocalypse in the first place. Prior to the events of the game, he was a self-made billionaire who spent his entire young adulthood fighting in a corporate world where ambition and cunning were tools of warfare. In a sense, he can be considered a kind of antithesis to a character like Andrew Ryan. Both built a fortune from hard work and ambition, but where a character like Ryan would internalize his success to inflate their self worth, and in so doing come to love capitalist principals and human endeavor to the point of making it almost a religion, Hikawa found himself disgusted by what he saw. To him, the human race had begun to destroy itself due to individuals allowing their ego to constantly inflate their importance above the rest, regarding their confidence as unwarranted arrogance, and a lack of ability to understand others.
As he came to this conclusion about the world: that it had become stagnant due to the unchecked greed and corruption of the individualistic human race, he learned of the cycle of death and rebirth the world could undergo, and set into motion the chain of events that lead to the death of the old world so that he could create his ideal world in its place.
Hikawa represents the most law-oriented of the Reasons, "Shijima". Shijima, as he envisions it, is a world of tranquility and silence. Individuality diminished, and the flame of human ambition controlled so that it would not become a conflagration that destroyed everything for the benefit of the arrogant. In the world of shijima, all people coexist in flawless stillness and harmony, sharing a collective inner peace and equality with all parts of the universe.
Since he is the only human who survives the apocalypse who also knows in advance about the Reasons and the cycle of rebirth, he has a considerable advantage over the others, quickly amassing an army comprised primarily of demons and evil spirits and establishing a base at the tower long before the player reaches it. (It's interesting to note that the demons who occupy hell and are associated with evil primarily join the reason of Shijima, since they're exhausted from the constant chaos of the world they used to inhabit, and long for the peace that Shijima would bring, while Angels and other holy demons joined the reason of Yosuga, because they were weary of order and longed for chaos... but I digress... this post is long enough as it is. DX)
As I mentioned in the first place, his ending being disastrous for him isn't directly mentioned if you ally with him and obtain the shijima ending, but this is only because the Shijima ending concludes just before the new world is established. In all routes where you do not ally with Hikawa, you must fight and kill him to climb the tower. It's when he lies dying that he realizes a truth about his Reason that he had not been consciously aware of before: Shijima would not simply control the fire of human ambition, it would snuff it out entirely. The world he wanted to create would rob humans of an essential part of their identity, and thus would instill in them a subconscious horror that could not be dispelled.
Granted, all of that was confirmed in supplemental materials, with the game being much more subtle about it. Throughout the entire campaign, each time you speak to Hikawa he complains about the "noise" of the human world and how much he longs for silence. As he lays dying, and glimpses his ideal world one final time, all he has to say is, "It's so quiet... It's... It's too quiet."
So even in the ending where he wins, he would ultimately have that moment of fear... and then no emotion at all, for he created a world where emotions would serve no purpose.
*whew* sorry for the word count there... maybe I should have put it in a spoiler tag just to save space.