What demon truly means and why they changed his meaning?

 Demon is a word coming from ancient Greek language, in which it has a completely different meaning. In Greek it's Δαίμων > Δαι means give, share (Δαίομαι), so demon is the beings, which are sharing or give fates to the people base on their karma of course and justice above all. In Hellenism each human have his own demon, who is protecting him and guiding him, if he listens to his demon (intuition). They are powerful beings, who are trying to keep justice in our world and it has nothing to do with evilness.

The reason they tried to give demons a bad meaning, it's because during mediavel times, Abrahamic religions wanted people to not be connected with nature, neither with their intuition. They wanted people to think that it's something bad. But we see etymologically that it's something good actually. People definitely should follow their (inner) demon, if they want to level up in life. 

In Greek: https://el.m.wiktionary.org/wiki/%CE%B4%CE%B1%CE%AF%CE%BC%CF%89%CE%BD#%CE%91%CF%81%CF%87%CE%B1%CE%AF%CE%B1_%CE%B5%CE%BB%CE%BB%CE%B7%CE%BD%CE%B9%CE%BA%CE%AC_(grc)

In English: https://en.m.wiktionary.org/wiki/%CE%B4%CE%B1%CE%AF%CE%BC%CF%89%CE%BD

So from now on, you know what demons truly are and why you shouldn't be afraid of them, but instead you should be afraid of people, who are changing word's meaning to serve an agenda.