Let's talk how the government kinda works a little bit and why there is unnecessary oppression

So the society of daemons under the Toriel's reign values strength emotionally and physically. It is very war-based in its ideals. Unfortunately, they aren't under war and haven't forever. Technically they are also fed propaganda about humans and such as well but not entirely at the same time.

Toriel is very controlling. She believes her way is the right way and the only way. The more unified she can have the daemons be, the better. Unfortunately, she is pretty far removed from her caring UT self after centuries of bitterness and grudges. The power from having not only a human soul, but a sacred stone as well has poisoned her heart and just fueled her hatred. It's also given her a bit of a power craze as well. The reason she has not gathered the other stones is because she is afraid of the consequences of taking the other stones herself. But a human SOUL that has already absorbed the other stones? Now that she could get interested in...

This is also why she will fight Baby. Not only does she want their SOUL, but she cannot stand the idea of a human breaking the barrier for them. It has to be her. She sees no other way.

Let’s think for a moment. Why would there have been the first monster revolution? When did Toriel become “cruel”? Asgore simply fled because Toriel killed his children. What was the kingdom like before that moment though? After all it happened a century or so prior to Baby, and Toriel ultimately felt it necessary to crush. Why did other monsters end up following Asgore though? What horrible changes happened in the time between Asgore leaving, and the second monster kingdom forming? I can’t double check the timeline at the moment, but I believe the time may be too short. The kingdom can’t just dramatically worsen over night. However, with the king leaving tensions should definitely rise.

So, kingdom during Frisk fall; it’s tense and a little split. Frisk can mean peace or war. They’re a human, humans trapped them there. But Frisk is a child, a kind one. And they can’t even wield the sacred stones, so what would their death solve? Thus, most monsters are tolerant. Some are bitter, but know protest won’t get them anywhere. Others are fully accepting, but largely are involved in the Castle itself.

But soon; Toriel has to deliver the news of the loss of both the royal children and king at the same time to a kingdom of monsters who feel hopeless. So, she lies. She says Frisk killed Asriel, so she took their soul in retaliation. It was an act of war. She also says that Asgore fled as a coward, not believing what he saw with his own eyes. He was a “lunatic”, and now Toriel would lead them to greatness as the new king. They would instill fear into the hearts of humans, in fact, they would become just what humanity thought they were. DEMONS. (Yeah let’s switch to demons now it’s easier to spell)

Monsterkind was divided. Many of the Castle workers knew what truly happened, some of the other monsters spotted Asgore fleeing in a different manner. The Castle workers that knew the truth were disposed of or forced to secrecy. Toriel became much more controlling and much more “military” focused. This is what began the downfall of the kingdom, Toriel’s bitterness. Some monsters embraced it at first, but over time it became more and more oppressive and restricting. Asgore began to take in monsters to the old abandoned capital, thieves, crooks, anyone who wanted. This is what became the first successful separate kingdom. But, Toriel would eventually find out who’s in charge and prepare to get rid of it.