Have you seen a real demon possesion? I have seen one in the Philippines, a woman had enormous strength, 5 men could hardly control her.She was possessed after her family brought her to a quack doctor or faith healer. (This is rampant in the remote parts of the country.) She is not a normal person at all. She is angry with Christians and with those who pray but not angry with others. She can spot who is her enemy in the crowd.
I see Westerners possessed with all kinds of things. One man I know is possessed with sexual pleasure; he now cannot look on a physical body without thinking of it in terms of the arousal it brings him. It is very sad. He needs to know the truth as it is in Jesus. The Lord seems to allow such things in order to show the slavery and depravation which follows from belonging to the kingdom of Satan.
The Bible provides the revelation for understanding the phenomenon of evil. It never leads us to believe that there is some kind of paranormal activity which influences the actions of human beings. It always insists that humans are responsible and will finally be judged for their own actions, whether they be good or evil.
Concerning your observations of "demon possession," I can only say that phenomena require principles in order to interpret them correctly. Because you accept the idea of demons exercising a physical power through human experience you interpret these experiences accordingly. From my own perspective, I do not believe the woman demonstrated a physical strength which was not natural to her. Rage is a powerful passion. Enraged people often do not know their own strength.