Ren is correct, the origin of demon is daemon and was defined as a lesser god. Later, the Romans took the word and changed it to demon. Since Christianity considers that there is only one true god, all deamons/demons were classified as being wrong to work with and therefore evil. The latin translation of demon (de mon means "of evil"). That is why many higher beings, gods and goddesses of various cultures were classified as "demons" in "demonology". If you look into the original origin of many so called "demons" they were in fact ancient gods of Greek, Sumerian, and Babylonian cultures to name a few.
I studied/study latin as well. =)