Most authors who wrote theological dissertations on the subject either truly believed in the existence of infernal spirits or wrote as a philosophical guide to understanding an ancient perspective of behavior and morality in folklore and religious themes.
I leave it in the middle if demons exist as independent beings who walk our reality but feel rather sure that most people are familiar with the existence of some demons that they struggle with. They mostly consist of some irresistible auto-destructive drive like substance abuse, promiscuity, gambling, and many other vices that ultimately can lead to the physical and moral bankruptcy of a life.
In my opinion the true demons that walk this planet and that are not exclusively rooting into our own mental processes, are the demagogues that incite people to commit morally questionable acts while letting them believe they´re doing the right thing. You find them prominently among cult leaders, politicians, and managers of multinational corporations. Their power to disturb the moral compass of a mass of followers can sometimes astonish a thoughtful observer.
A second category consists of the people with a damaged mind. Here I´m referring to the psychopaths, sociopaths, and schizophrenic. These people have a broken moral compass and are driving other people into ruin to satisfy some alien mental disposition.
Finally, we sometimes perceive demons where there are none. Like some youngsters demonizing the older generations for all the social and cultural imperfections they perceive, or the left and right wing politicians mutually blaming each other for a lack of common sense (while the truth usually lays in the middle, but polarization is easier to sell to a voting public), or the eternal disputes between men and women about whom has the right perspective upon the reality (while both views have their merits and shortcomings).