I used to believe that Demons are simply fallen angels, but I have a problem with two texts:
Jud 1:6 And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day.
2Pe 2:4 For if God spared not the angels that sinned, but cast them down to hell, and delivered them into chains of darkness, to be reserved unto judgment;
The angels that fell are either in Hell and chained up or they are not. If they are, they can't be running around possessing people.
The first thing to note is that these are not all fallen angels, but a certain group "during the days of Noah", and they were not sent to hell (i.e. Greek "Hades"), but to "Tartarus". We see this similarly depicted in the Greek mythologies as the Titans who rebelled against Zeus. Furthermore, the term itself in the Greek e.g.
δαίμων daimōn
From etymology online:
demon (n.)
c. 1200, "an evil spirit, malignant supernatural being, an incubus, a devil," from Latin daemon "spirit," from Greek daimōn "deity, divine power; lesser god; guiding spirit, tutelary deity" (sometimes including souls of the dead); "one's genius, lot, or fortune;" from PIE *dai-mon- "divider, provider" (of fortunes or destinies), from root
*da- "to divide."
From Vine's Expository Dictionary of New Testament Words:
Demon, Demoniac:
not a diminutive of
daimon, No. 1, but the neuter of the adjective
daimonios, pertaining to a demon, is also mistranslated "devil," "devils." In
Act 17:18, it denotes an inferior pagan deity. "Demons" are the spiritual agents acting in all idolatry. The idol itself is nothing, but every idol has a "demon" associated with it who induces idolatry, with its worship and sacrifices,
1Cr 10:20,
21;
Rev 9:20; cp.
Deu 32:17;
Isa 13:21;
34:14;
65:3,
11. They disseminate errors among men, and seek to seduce believers,
1Ti 4:1. As seducing spirits they deceive men into the supposition that through mediums (those who have "familiar spirits,"
Lev 20:6,
27, e.g.) they can converse with deceased human beings. Hence the destructive deception of Spiritism, forbidden in Scripture,
Lev 19:31;
Deu 18:11;
Isa 8:19. "Demons" tremble before God,
Jam 2:19; they recognized Christ as Lord and as their future Judge,
Mat 8:29;
Luk 4:41. Christ cast them out of human beings by His own power. His disciples did so in His Name, and by exercising faith, e.g.,
Mat 17:20.
Acting under Satan (cp.
Rev 16:13,
14), "demons" are permitted to afflict with bodily disease,
Luk 13:16. Being unclean they tempt human beings with unclean thoughts,
Mat 10:1;
Mar 5:2;
7:25;
Luk 8:27-29;
Rev 16:13;
18:2, e.g. They differ in degrees of wickedness,
Mat 12:45. They will instigate the rulers of the nations at the end of this age to make war against God and His Christ,
Rev 16:14.
Demon, Demoniac :A-1: a demon, signified, among pagan Greeks, an inferior deity, whether good or bad. In the NT it denotes an evil spirit. It is used in Mat
www.blueletterbible.org
One other reference which I am still looking for points out that it refers to being "sent from above", and doesn't seem to carry the negative baggage it accrued over time.
Actually the Bible never actually tells us the origin of Demons as far as I know.