Writing a novella about the subject of ‘monster’, which you can think of as a short novel. Longer than most short stories, but shorter than a novel. Anyhow, need to see how different people perceive a person as a monster, that’s all.
Ah-cha! That is the key perspective that helps me understand the kind of answer you are looking for.
Communities and individuals both might judge another potential member or peer a 'monster'. They key part is that the person be acceptable at one point, (perhaps before meeting their judge though usually not) and lose that acceptability in such a way that redemption seems hopeless or 'monstrous' in and of itself.
Violating ethical or moral codes usually are part of this, or just living in a manner different than the prescribed social codes. Someone too short in a tall community might be enough to do this in an extreme enough manner, but in most social codes adjacent to our own, this means murder without acceptable motives, direct action against people of a different peer group, or endorsing those previous things. Even then, 'monster' ultimately a personal moniker from one person to another.
Its hard for a third party to tell X that Y is a monster. X has to ultimately make a judgement on Y personally.
Its an interesting question. I did see that you phrased it to ask about "if a person was extremely intelligent" and that really depends on what the person does with being intelligent? Are they a smug asshole that identifies that others need to suffer for their own or the group's gain? I mean, some of the people might call them a monster if "ends justifying the means" is not a shared cultural ideal. Even then, its hard to justify that sort of thing
to the people suffering or dying.
This is why people hate the 'trolley problem' and the memes that spawned from it. Without context, its just a useless measure, reality doesn't work with such absolutes. Fiction, however, does.
Ultimately, however, you can't account for if your *readers* will see the person as a monster because readers come from such a broad diaspora and, just like the trolley problem, know that this is a fictional situation. If the people sympathize with the person on the track who has to die, the person with the controls is a monster. If they sympathize with the controller, they are not. The ending is preordained. The character could fail even, grinding everyone including themselves to dust, and some readers will thing the attempt was worthwhile, and not monstrous. (Some might think you, the author, is monstrous instead, for having written such a futile story.)
Interesting focus, hope you find your answer among all the replies!