Demon is derived from the Greek term Daemon, which literally meant "wise spirit". 
Daemons weren't regarded with evil (the name was used for all spiritual entities) 
until the thirteenth century. St. John connected the term daemon with the "fallen 
angels". Many of the so-called "fallen angels" are in fact gods of older religions. 
For instance, the Persian archangel Aesthma became Asmodeus and the Phoenician goddess 
Astarte became Asthareth or Astaroth (some see these as two different entities, some 
list them as the same entity. Depends on the hierarchy used and the person you are 
dealing with). Demonolators worship/work with these dark aspects (and sometimes, 
not-so-dark aspects) because they represent the part of man he has yet to understand. 
We do not see demons as "warring" with a separate breed of spirits called angels.