What is a inner demon?

An inner demon is mostly considered as in "You are your own worst enemy"

It could be explained with "Skeletons in the closet"

If you have the urge to do something bad, then this can be considered an Inner Demon.

An inner Demon is often something you think about too much. If you tried to save somebody from drowning and did not succeed, then this can haunt you for a long time becoming your "inner demon".

I suppose the entire Angel / Demon thing came to life to actually try to explain what was happening when people did really bad things and really good things.

As somebody possibly could say:

I don't know what hit them, must have been Demons at work.

She's such an Angel. Never do anything wrong.

It is a paradox though that When a person does something good, then she/he is compared with an angel. On the other hand when a person does something bad, he/she is most often not in charge because the demon did the work through him/her