Most people are aware that Evil Is Not a Toy, but there are a few that will go through the elaborate Summoning Ritual anyway and end up becoming a host for some demon or other, but in a few settings, making a deliberate Deal with the Devil isn't necessary for the demon to take control. The demon only needs a victim with the right (or wrong) mindset. More specifically, this is the idea that being an evil person leaves you vulnerable to the influence of evil beings. There may be specific types.