What is the difference between “the ego” and “the mind demon”?

If you are referring the Freudian concept of the Ego, Id and Superego;

The Ego is the self (such as personality), the Id the desire (such as libido), and the Superego is societal pressures and restraint (such as guilt).

The Superego is sometimes referred to as the conscience.

The Superego and Id conflict constantly (for example, your Id once seeing someone attractive would want to have sex, but your Superego would stop you because sex in public is socially inappropriate, and the person may not be consensual to sex).

Stubbornness, selfishness and inner frustration may be aspects of the Ego, such as personality, or may come about due to the Id overriding the Superego (such as selfishness), or the Superego overriding the Id (such as sexual repression).

Ego is more of the opinion of how much you are morally worth. If it gets too big, you value yourself too highly. Too low, you think you are not worth much.

The "Mind Demon" as you say, is exactly what you described.

So they're similar, but not the same. I'd say the "Mind Demon" is more closely related to a big ego, but not to egos in general.

Mind is broader than ego, ego is one of the principal component of the mind often assuming a preeminent role that would be detrimental to the individual such as twisting all sensations, feelings/emotions and thoughts in the mind to its own mythical identity relying on which spells our doom!