I realized today that when I used the word "demons" I was thinking of evil spirits of all varieties, including human beings that work evil from the spirit world. But others believe the word "demon" only applies to non-human spirits such as fallen angels or perhaps evil deities of some sort.
In the Bible, Jesus refers to the "unclean spirit" [or spirits] that possessed the man who called himself Legion because he was tormented by so many evil spirits.
[Jesus said] “Come out of the man, you unclean spirit!” And Jesus asked him, “What is your name?” He replied, “My name is Legion; for we are many.” And he begged him eagerly not to send them out of the country. Now a great herd of swine was feeding there on the hillside; and they [the spirits] begged him, “Send us to the swine, let us enter them.” So he gave them leave. And the unclean spirits came out, and entered the swine; and the herd, numbering about two thousand, rushed down the steep bank into the sea, and were drowned in the sea. (Mark 5:9-13)
But elsewhere, Jesus heals various people possessed by "demons."
That evening they brought to him many who were possessed with demons; and he cast out the spirits with a word, and healed all who were sick.
I'd like to know more about how various Christian denominations and other religions view those who do evil in the spiritual realm, and especially: do they believe that the term "demons" includes evil spirits who were once human beings.
Rather than personal opinions, a good answer will provide the perspectives of several religions or experts. I would not rule out spiritualists in the latter category.