Underfell is considered a community-made alternate universe setting of Undertale. Rather than being defined by a particular author, it’s defined by a premise and commonalities in character interpretation. Different people make different interpretations of Underfell, but, generally, it’s like canon Undertale except monsters are “evil”: either literally or just meaner, more cynical, and scarier-looking to the point they seem simply evil at first.
In some interpretations, the setting’s different from the start. In others, it’s Asgore who causes the divergence into scariness and cynicism. Yet, the cruel, cynical setting of Underfell need not be caused by an overt act of evil. Oh no. It’s quite plausible the setting can come from one act of mercy.
One Change is Necessary
That one act of mercy is…modifying his declaration of war so fallen humans are made prisoners of war, rather than killed. Remember, Asgore’s a big softie; he only declared war on humanity because he was blinded by rage, rage that came from the grief of both his kids dying on one day.
This is pretty plausible. He certainly has the motive to do it: his reluctance to kill humans, give his people hope, and his desire to get back with Toriel. (who left him in disgust over his declaration of war) One need only make him slightly less of a coward and slightly smarter to do this, or, perhaps, get inspired by his royal scientist on the difficulties of making soul power synthetically. Though he’s too much of a coward and a “big softie” to take away the declaration of war that gives his people so much hope, perhaps he’d modify it and make post-hoc justifications.
Potentially, Asgore’s personality could be kept unchanged: he might make various accommodations for the humans that keep them comfortable, but have plausible excuses related to the safety of keeping humans, their use in experiments, and controlling the SOUL supply with a breeding program (or alleged program).
Details on Excuses
Asgore could claim having live humans, rather than dead ones, would be much more useful for studying human SOULs and learning to create SOUL power synthetically. The idea of getting to a godlike state of seven human SOULs quicker or even having an army of SOUL-enhanced monsters would be very appealing to the populace, after all.
He might also claim humans can reproduce for a big chunk of their lifespans, so, even if it’s decades before another human shows up, once there are two they could establish a breeding program to control human SOUL supply. (Not that he’d force them to breed; this is just something he’d say to try to justify not killing them)
Furthermore, theoretically being able to study the development of human SOULs throughout the lifespan might give crucial insights on creating SOUL power synthetically.
Cushy “prisons” (potentially in name only) could be excused as a measure to prevent the prisoners of war from getting too aggressive. He could say: “humans are very dangerous, you know, and sub-par environments might drive them to murderous intent”. He could even keep all the fallen humans together, under the pretense it’s useful for the breeding program and more efficient to keep all the human experts in one place.
He could argue that having all the guards concentrated in one place would prevent escape attempts better than having them scattered thinly across multiple facilities. (it might be cheaper, too, if the people are concerned about royal treasury funds being used wisely) Speaking of the guard, he could even excuse having minimal guards within the “prison” facility itself: one wouldn’t want humans to get too scared, given what they did last time they got scared of monsters.
Asgore could even visit the fallen humans, and say that the plan for monsters’ freedom is so important he wants to ensure everything goes smoothly.
Asgore could say that, as he is very strong, it will be helpful for him to show up to re-capture escaping humans or deter escape attempts through unpredictable arrivals. If anyone debates whether he’s strong, he could always bring up the fact he’s one of the few survivors of a one-sided massacre of a war, and few would still be around to clarify the circumstances.
On account of living on the Surface before1 and raising a human child, he can also tell them he has a lot of experience with human psychology.
Indeed, he even has a fairly plausible excuse to bring them on field trips: the people would love to see evidence of the captured humans. Also, humans need exercise to stay healthy; keeping test subjects healthy is standard procedure. Asgore could even let them see the barrier: it was worth checking, given the whole “a powerful SOUL is needed” deal. If they’re on a leash and under guard, it’s fine.
In fact, he could probably make stuff up (within reason) about human breeding requirements just as an excuse to give them better lives; it’s not like anyone else has firsthand experience or are going to correct the king.
Politics of Human-Killing
As Toriel points out, Asgore could easily have just killed one human, absorbed that SOUL, and crossed the barrier to obtain more through peaceful means. (e.g., negotiating to obtain the SOULs of recently dead humans or prowling through graveyards)
The number of SOULs monsterkind has collected is apparently common knowledge among monsters. It is possible that, in their admiration, they think the beloved king is so wise that he has a good reason to not absorb just one SOUL, or, for that matter, six.
Yet, by delaying fallen humans’ deaths inevitably without retracting his declaration of war, monsters might get suspicious of his true motives and devotion to his people. Indeed, the more (publicly-known) accommodations2 he gives the humans, the greater the population’s suspicion of his true loyalty. The longer he goes without significant progress to freeing them, the less hope monsters would have. In turn, they might become cynical: people that go politically unrepresented for long enough, after all, may become cynical and despairing.
Things get more complicated if one of the humans actually killed a monster or two. Asgore might then forbid killing the murderous human, either in revenge or as a reasonable precaution because of the human’s likelihood to kill again and acquisition of LV.
Note that, if The Human killed fewer than ten monsters in the Neutral Run (and befriends Undyne and Papyrus), Toriel takes the throne and institutes a new policy of non-hostility towards humans. If, however, The Human killed more than ten monsters (or Papyrus), the monsters rebel, and Toriel’s either kicked out of the castle (by Undyne) or resigns peacefully. So any evidence humans are violent and a threat to monsters will likely lead monsters to try to overthrow Asgore.
Of course, the people might be restless and violent even if the humans don’t kill anyone: Undyne is so certain of her belief humans are basically evil that she’ll try to kill a human child who has done no harm. Some monsters are clearly delighted at destroying humanity, so this anti-human prejudice might be common among monsters.
If the Royal Guard exists more to fetch humans and recover escapees than to kill them (or bring them to the king to kill; their purpose is unclear), civilian monsters might simply take matters in their own hands and become human-killing vigilantes.
In fact, if a human-killing vigilante tries to kill an escapee human and the human understandably defends themselves, this might be seen as evidence the humans (or at least the specific one) ought to be killed.
Official human traps (say, around a facility) might also be secretly modified to be lethal, or monsters might add further, lethal traps beyond authorized traps.
All this proves it isn’t necessary for monsters to be more evil or mean from the start to cause Underfell. Underfell could logically (though not inevitably) happen from just one tweak in the timeline and reasonable in-character reactions to that.
Note that this more merciful approach still has plenty of opportunity for pain and darkness, beyond the political unrest and roaming vigilantes.
To keep up Asgore’s idea, the human facilities would have to conduct extensive testing of the humans, even painful SOUL experiments.3
Assuming a cynical, anti-human populace, the facilities would need to be heavily guarded, even if only on the perimeter, and there might be clashes between the facilities’ guards and the general populace.
Monsters might become human facility personnel just to try to kill at least one human, potentially making the humans distrustful of the personnel.
Toriel might still try to keep humans in the Ruins even with Asgore’s modification to the war, so she can protect them from threats and painful situations both in and outside human facilities.
Generally, it’s assumed the king in the intro was Asgore. The history’s a bit muddled, though. ↩︎
He might seal fallen humans in the sparsely-populated Ruins for a while, so they won’t have as many targets if they are violent. But if he knows Toriel lives in the Ruins, that’s a risk: they might kill her, and if they absorb her SOUL, they’d become even stronger and get past the barrier. Conversely, if he changes the declaration of war before any humans die, Toriel might hear of it, learn they’re kept in reasonably good accommodations, and then get back with Asgore….so she might not be in the Ruins when a human falls. ↩︎
In some fanfictions, SOUL experiments are painful. Where this is the case, the king and queen might forbid painful SOUL experiments out of concern for Chara’s physical or mental well-being. ↩︎