It’s a movie. Media likes to play off of popular trends, tropes, and cliches. So, demons trying to hurt people is essentially one of the most popular tropes. You need to understand that anything you see in media is just there to entertain. It’s not real. There are times where they borrow from actual lore, but they always twist it for the purpose of entertainment.
I don’t know what you read, but from my own collection, this is what I’ve gathered. According to ceremonial magic, particularly the solomonic and grimioric traditions, demons are earthly spirits lower than angels. They are often times described as being chimeric in appearance, and can be good or evil. However, I’m beginning to wonder if the use of “Good or Evil” means being cooperative or uncooperative with the summoner. It doesn’t necessarily mean having benign or malignant intentions.
In many Left Hand Path/demonolatry books, what we call demons were originally the gods and spirits of older polytheistic societies. Though, when the Abrahamic religions conquered them, the gods and spirits they worshipped became demons. They were conquered, vilified, and subordinated by the Abrahamic god. Which fits with the Solomonic definition of demons being inferior spirits.
Basically, demon is a very broad term that can refer to any spirit outside of the angelic hierarchy, just as how Pagan refers to anyone who wasn’t a Christian, Muslim, or Jew. That fairy that lives in your garden, that’s a demon. That elf who dwells out in the woods, that’s a demon. That Naga that guards a particular lake, that’s a demon. That dragon who brought you into the astral plane, that’s a demon. It’s not going to be some horned fiend from the underworld who plans on doing a bit of mischief. It can also refer to spirits of nature, or even the higher celestial realms. Though, yes, we do accept a more specified definition for the term, and have created a specified pantheon of high-ranking demons.
Hell, one might be willing to think the term is more of slur, like the spiritual equivalent of the N word. I am on the hunt for a more appropriate word for an underworld spirit, btw.