It might be. What people referred to in ancient times as a "demon" might better be referred to as a very negative thought form which takes a person over at times and causes them to behave in destructive ways. A modern therapist probably wouldn't call it a "demon", but might call it a "complex" or an "obsession" or some other current popular term that fits the modern prejudices and assumptions.

If a fit of anger takes a person over and causes them to do violence, then they have been possessed by something stronger than they know how to deal with at that time.

The only real question is, did it originate solely within that person's own consciousness or is it due to the influence of an outer agent in the form of a negative thought form that affects or enters the person? We have no way of being sure about that at this time. We only have our standard cultural prejudices, our standard prior assumptions, and our standard favorite terms to describe the situation. The terms change as the culture changes...but the problem of the afflicted person remains the same in its outward effects.

No one can say whether the "demon" is self-manufactured or not. We can only have rock-solid opinions based on our prior assumptions of what is "real" and what isn't...or admit that we don't know for sure...which would be a bit wiser, in my opinion.