Where do Hell Guards fall?
Yes, they are Asuras. They are not created. They are also hell beings who become stronger from time to time due to their past good Karma and assume a position of command. At other times, they suffer like the rest.

They are not hurting people(humans). They are hurting beings who are at the bottom of the scale of virtue. So the Karma they get is comparatively less. In any case, they don't get any opportunity to do good. So, unless a past good Karma resurfaces, they have to wait till the end of the world to escape from hells.