What is more terrifying?

In games, that's how it is, but in theory, a zombie doesn't die AT ALL - that is, the only way to stop it is to chop it up into pieces so it can't do anything.

Are you talking about in real life?:D
Probably, but it turns out that a zombie is the ultimate weapon? An ultimate soldier?
No, there still need to be some genre conventions. Let's say that infusing the entire body with magic is very expensive, so a necromancer mostly revives only a semi-decayed brain, which moves the body's muscles with residual nerves. This explains the awkwardness and some movement problems where there are no muscles or nerves left;)
Chop off the head - save the world.

And it's easy to revive a skeleton - it's like a kind of autonomous telekinesis. It can't move on its own. There's nothing to move with! So, it's enough to break it a little, and the magic will leave the bones.
I think so:)

It will take several years to regroup, and in that time, it will be possible to destroy all the demons and close the portal.

Not necessarily. Besides, specific material is needed for the undead, and there are countless demons in hell; it's easier with summoning.

Demons probably take too long to regroup; they lose their bodies, and only the spirit goes to hell. It is unknown how long it will take to get a new material form.

As an option - to inhabit a stupid mortal.
And Smaug wanted not only to eat but also to rob, kill, and defile women.

>They are simply not kind to people who are not kind at all.

Killing and robbing anyone is not good, whether it's a person or a cockroach. And they also kill good people.

Added 2 minutes ago
...and even defenseless peasants...