A demon is something scary that lives inside of you. I think it's an OK metaphor.
edit: really - addictions don't live inside you, perched on your heart, making you do bad things. They are not different than you.
edit: really? Happy to accept the downvotes on this one.
That's what the demons want you to think.
A homunculus didn't do it; I am it and I did that.
Many of your behaviors are effectively semi-autonomous reactions perched inside you.