Ok, here is my opinion. I believe that demons were around before man (of course). I believe that they are beings from other dimensions or parallel universes or what not that have found a way into our world/dimensions and have had a huge affect on mankind. I believe they are capable of both good and evil. There are many names for demons and many different beliefs. And don't get me wrong, I respect others beliefs and point of views about demons and wont dismiss the possibilities of other beliefs, if that makes sense. I am a very open minded person. I will consider all the possibilities. But above is my personal belief.
I believe they have free will to do good or bad, They choose to be good or bad towards people. Just like we can be good or bad towards people. IMO Demons can be misunderstood sometimes. I believe they can do acts of good as well. Can you imagine a world with out demons? As I said before they have had a huge impact on mankind.