Short answers, pulled from the story.
The word demon derives from the ancient Greek daimōn, which denoted a spirit or divine power with no negative connotations. It was used by Plato to describe the divine inspiration of Socrates. The negative meaning entered through the Septuagint translation of the Hebrew Bible into Greek, which rendered foreign gods as daimones with implied evil, and this usage was then inherited by the New Testament.
Large portions of Jewish demonology, which became a key influence on Christianity and Islam, originated from Zoroastrianism and transferred into Judaism during the Persian era. Zoroaster demoted the old daevas to demons by elevating Ahura Mazda to a single supreme God. The dualistic cosmology of good versus evil, with Ahriman as the principle of Evil and creator of demons, shaped the apocalyptic literature that in turn built the foundation for the Gospels.
Ancient Egyptian demons divided into guardians and wanderers. Guardians were tied to specific sacred places and protected the gates to the netherworld; they could be benevolent toward those with the right secret knowledge. Wanderers were associated with possession, mental illness, death, and plague; they could arise from the world beyond creation out of pure evil motivation, without divine instruction, and could be warded off but never destroyed.
Demons appear 55 times in the New Testament, with 46 of those references concerning demonic possession or exorcism. They are depicted as adversaries of Jesus, explicitly ruled by the Devil or Beelzebub, and as causes of pain, suffering, and illness rather than as tempters. Some early Christians, including Irenaeus, Justin Martyr, Clement of Alexandria, and Lactantius, believed demons were ghosts of the Nephilim, the giants described in intertestamental writings.
In Islamic belief, jinn are morally ambivalent beings, ephemeral and shadowy, primarily linked to magical practices, who share attributes with humans including mortality. The šayāṭīn, or devils, are malevolent forces that actively obstruct God's will, bear less resemblance to humans, and are used synonymously with the Persian term Dīv. Al-Ghazali described šayāṭīn as operating in the realm of mental processing while fiends operate on the sensual level, forming a cooperative assault on human virtue.
M. Scott Peck, an American psychiatrist, initially investigated exorcism in order to debunk possession as myth. He described his findings in Glimpses of the Devil: A Psychiatrist's Personal Accounts of Possession, Exorcism, and Redemption, reporting that two cases in his practice did not fit any category known to psychology or psychiatry. He concluded that possession was a rare phenomenon and that possessed people were not evil themselves but were doing battle with evil forces.