Certainly people are responsible for the sin which allowed this demonic activity within, but they are driven past what they intended, as the evil personality takes over their actions. Some children are born under a curse and have demons that cause their erratic behavior. Parents are in ignorance as to why some children beat their heads on the floor in rages, scream and yell uncontrollably, bite themselves, or have continual nightmares. Many children do have demons and need to be set free. And he said, Of a child.
Demons can attack with no sin involved on the victims' part as in the case of children.
Demons can also enter when the person's will is weakened and he is overpowered by the devil. This happens many times when a person goes through a terrible shock or experiences an incident that produces extreme fear. Accident victims who are knocked unconscious due to blows on the head often are subject to demon invasion. Addicts and alcoholics eventually have demons as the drugs and alcohol suppress their wills leaving them open for attack. Demons are invited in by persons who yield to sin continually.