What makes a mild mannered, promising young scientist decide to arm himself to the teeth, walk into a suburban movie theater and start killing innocent people at random?

Did he have a bad childhood experience? Doesn't seem that way. Everybody says he was a nice, very smart, shy kid. Maybe a bit of a geek, but there are millions of geeks who don't go on killing sprees. Was he insane? There weren't any warning signs that he was crazy. Was he politically or religiously motivated? No evidence. Was he driven crazy by violent video games? He didn't seem to play more than most kids. Was his mind poisoned by the nihilistic second Batman movie? It was a dark film, but millions saw it and didn't go on a rampage. Was he somehow "possessed" by the Joker character? Some say the demonic Joker drove Heath Ledger to kill himself.

Now it's getting spooky. There was a weird phone message with bizarre guttural voices and moans. Was he demon possessed? Maybe. It happens.

Demonic infestation is a rare, strange and terrible psycho-spiritual affliction. In simple terms, a malevolent, separate intelligence infests the mind and spirit of a person. It takes over the rational faculties and dominates the personality. The phenomenon is real, but anyone who has ever dealt with the problem realizes that the demonic realm is complex. The human person is an intricate organism in which the physical, mental and spiritual aspects are totally interwoven. Therefore, in most cases, trying to diagnose the possibility of demonic influence is extremely difficult.

This is because, in theory, demonic influence in a personality can exist on several different levels. Experts disagree about the terminology and extent of the diabolical influence, and in this arcane discipline, for reasons that will become clear, there are few set rules or guidelines. However, some levels of demonic involvement can be observed.

The first level of demonic influence is temptation. This is not just the mild desire to drink too much, overindulge, or have a sexual dalliance. That's just part of being human. If there is a demonic element, the "temptation" is to do something radically and extremely vile. This can happen in a person's life without any sign of supernatural activity.

The second level of demonic influence is obsession. At this level, there is still no sign of anything paranormal happening. The person starts to give in to the temptation. He may become reclusive and secretive as he becomes obsessed with the evil that he is entertaining. This evil may be in the form of occult activity, violent video games or movies, pornography, drug abuse, sexual perversion, sexual promiscuity, or obsession with power and violence.

The obsession with evil will probably have an addictive element. The personality begins to change. The individual may seem "normal" most of the time, but he'll have "dark moments" when his "inner demons" take over. The difficulty in diagnosing demonic influence is that these same symptoms may indicate substance abuse problems, mental illness, social maladjustment, emotional inadequacy, relationship problems or a complex web of such difficulties. Demonic influence will cause these symptoms, but these symptoms are not necessarily a sign of demonic activity.

In the first two levels of demonic influence, the malevolent spirit is still outside the personality. On the third level, the malevolent spirit seems to enter the whole person in a spiritual, mental and even physical dimension. This level of influence is called infestation. This is the stuff of exorcism movies—when the person exhibits disturbing physical, mental, and spiritual signs of infestation. The individual is "taken over" by the demonic spirit and paranormal phenomena are exhibited. When the signs of preternatural strength are seen, horrible alien voices come from the person, vile blasphemies are heard and perverted and violent actions are witnessed, one can be fairly sure that a demonic infestation is happening. However, many of these symptoms may also be signs of a deep mental or spiritual illness which is not demonic in origin.