Most energy that is of a spiritual nature is just residually left imprints of those who lived and worked in the area. Just like a raw imprint of fresh laid energy lines for electricity there will be an impression that can be detected by monitors or pendulums.
Once the energy field of the human body leaves it collects itself into a ball (the most simplest shape) and it either moves into another dimension or stays in a familiar area. There is nothing mystical about the process. If the person has led a mostly decent life they have a fairly easy choice to either go where their loved ones are or stick around till they decide to be somewhere else.
For those who have lived a tortured existence their travel is harder to determine once they die. Much of where they think they should be has been taught to them throughout their lives and they may decide that there is nothing for them to go to. These spiritual bodies can stay in a loop of confusion for decades without finding peace. They are still not what I would call a demon.
The ones who cause massive deaths and chaos and then die for whatever reason still have to make a choice as to where their energy body will end up. If they have no set path they can and usually do stay in a chaotic loop that can attract more chaos and encourage continued patterns of destruction among the living. These areas are very dangerous to those who are dealing with torment of any kind. I do believe it is the physicalness of the energy itself that can draw people to it who are easily swayed, or deeply curious about the darker paths of energy work. Those who have died in this manner or caused death to happen are still not demons.
It is in my knowledge to this point that demonic entities were here long before the planets were formed. They despise anything created and wish to not only extinguish all life forms, but also destroy everything in its path. I do not believe that demons are qualified to a religion, or that there are any real distinctions except that there seems to be some sort of hierarchy as with all energy beings. There are also places on the Earth where this type of energy is very prevalent. As with all things there will be a certain percentage of people who will be attracted to inviting or hunting these extremely denser energies.
Most of the spiritual events that I have come across are those who were still attached by emotion, or disbelief that they infact are dead. Once given the opportunity to move on they usually do without further issue.
Not everything though is always picture perfect and there will be situations where if you are not prepared to take on a spiritual situation and you decide you're going to do it anyway expect that things may not go the way you think they should. Just because the energy is probably not a demon does not mean it will simply go away without first causing a few disruptions. Which leads to my next statement; never mess with things you do not understand.
Often times people will resort to lighting sage and throwing holy water at an energy source that is bothering them. It's not like the movies, or tv and often doing something like that can lead to unnecessary problems later. Trying to balance a situation without knowing the reasoning behind the tool is just as bad as trying to fix a car without ever learning about how they work.
Do your research talk with people who know about the area, take notes about the suspected imbalance of energy and most importantly learn what it is that you are dealing with to properly quiet the situation.