To many, the word "demon" is synonymous of horrid beast associated with something evil. This description, while being fairly correct, is more of a stereotype. In some books, it is claimed they once ruled the Earth, as humans now do, in others that they are intermediaries of Gods and humans. It is uncertain where it all originated from, but it's mostly to have emerged from the first belief of a higher being. Demons would often be the opposite end of the higher being, making it a battle of Good vs. Evil.
This makes demons, in the eyes of many, religious creatures, since they are claimed to be pawns of whatever force is opposing a good god. But these pawns can also be influence to a non-destructive nature, if the master wishes so. This makes the overall definition more of a general opinion.