Short answers, pulled from the story.
In ancient Greece, the word daimon described the divine inspiration of Socrates, a guiding voice that warned him away from error. It was a neutral spirit, a divine power that could be either good or bad. This concept did not begin as a monster under the bed but as a neutral spirit.
It was not until the Roman Empire and the rise of Christianity that this neutral term was twisted into a label for pure evil. By the early centuries of the Roman Empire, the numinous presence of the Greco-Roman gods was reinterpreted as the presence of demons. This transformation turned a word meaning divine power into a synonym for the devil.
The founder Zoroaster elevated Ahura Mazda to the one supreme God and relegated the daevas to the status of demons. In this tradition, demons became the children of Ahriman, the principle of Evil, and their existence was a test of free will. This belief system influenced the Abrahamic religions, particularly Christianity and Islam.
The idea of fallen angels emerged in the second and third centuries, as Christian writers interpreted biblical passages like Revelation chapter 12 and Jude 9. Augustine of Hippo established the position that demons are spirits who turn away from God. This view became the foundation for Western demonology and the Catholic Church.
In the Islamic tradition, demons are divided into two types: jinn and devils, or shayatin. The jinn derive from pre-Islamic Arabian beliefs and are created from fire and air, sharing attributes with humans such as mortality and the ability to possess people. The devils are malevolent forces akin to the devils of the Judeo-Christian tradition, actively obstructing the execution of God's will.
In Hinduism and Buddhism, the concept of the demon is fluid, often representing a state of being rather than an inherent evil. In Hindu cosmology, nothing is either purely evil or good, and even demonic beings could eventually abandon their demonic nature. In Buddhism, ghosts and evil spirits are not inherently evil but are reborn in states of misery as the result of bad karma.