Demons are strong beings on spiritual scale.No,you cannot assume that all of them are evil or good.Its best to respect who they really are.
Inner demons as what someone said before,are manifestations of the subconsciousness.The subconsciousness is capable of creating images depending on the emotion such as fear itself would be formed.
Basically,its like an alter ego but also a different personality of you.This is most often classified as possession which is actually not quite.
Of course,demons are real but wouldn't go that far to possess someone who isn't that worthy of their own level.
Sure,many can claim they have inner demons.It can be just their personality.
That's how much i can explain of the concept.Its up to you to decide. ;)