What is more terrifying?

And in Disciple, it seems that demons were also originally on a different plane of existence, and only a small group of them were on Earth. And they were trying to bring Besrezen to this plane...
And they are still trying...

Besrezen was originally an angel who, at the behest of High Phazer, created Nevendar - the world of Disciple. But, since even in paradise there are many envious people (angels of a lower rank), they sowed discord among the peoples of Nevendar, a war began, and everyone killed each other. High Phazer was angered by this chaos, and, without thinking, banished Besrezen to hell, along with his people-followers, who later became cursed.
That is why the race is called that, although it includes both cursed people and demons, who were already there before, but lived exclusively in the underworld.

...in general, what Arom said about the undead is true - most undead are just mindless automatons, obeying the will of a necromancer. The exceptions are liches, vampires, and the like. The goal of simple undead is to serve their master-necromancer, or to wander aimlessly around. Well, ghosts also scare people. "Noble" undead, i.e., those with consciousness, most often want to gain immense power and punish everyone.

And for me, only dragons are purely positive, and even with exceptions.

I hasten to disappoint you: most often dragons are greedy and cruel bastards who sometimes strive to become dracolichs and gain immortality along with unlimited power.

Do bandits fear the devil when they kill their victims? No, they fear the spirits of the people they killed. They don't think about getting to hell. They think about how not to meet the ghost (of the killed one). And finally, everyone is afraid of death. Everyone wants to live. And that is why people depicted death as a skeleton with a scythe, and not as a demon with cards in his hands, smoking a cigarette. Because a skeleton is scarier.

Bandits are afraid of meeting a guardian, not of going to hell, or of greeting a ghost.
People depicted death as an old woman with a scythe not because "a skeleton is scarier," but because death is associated with this - with a corpse that decomposes to the state of only a skeletal base. Everyone has seen corpses, but demons - only witches and fools, "who have no place in civilized society."

And I doubt that every senior demon or imp is a concentration of cunning, terrible power, and wisdom)

There are lower castes everywhere, you know) But those very demons who are busy whispering heresy into inexperienced ears, who are busy with corruption and concluding contracts for souls with ordinary mortals - they are something more than just some pathetic little imp;)
In any case, a demon can not only kill everyone, like some random lich, but also convert a fool to his faith. After all, I have never had to eradicate sects that worship the undead.
That is why a demon is much more dangerous and terrible than a zombie) A demon cannot be killed (=
He will only "go back to hell" in most cases. But if you cut off the head of a zombie or vampire, that's it.