I think there is confusion in the paranormal community between what we define as 'negative' and 'evil' entities.
'Negative' entities are the opposite of positive entities (duh.....). These include the likes of, for example, the spirit of a serial killer. There is negativity about them when they are encountered in our plane. This negativity crosses over with them on their passing. They still hold ill intentions and malevolence towards others, but generally, apart from causing trouble and fear, there does not seem much evidence to support that they can, or will, cause physical harm to the living. Were these people 'evil' in life? Absolutely, but in death, they are just an entity that just remains full of anger and hate.
Entities that are 'evil' have never lived in the sense, and demons are a classic example of this, although there are other classes of entities that also fall into that non-lived category (i.e. elementals). Evil entities feed off negative emotions, such as fear and depression, to make themselves stronger. They are, for a better description, an existence made up purely of the worse emotions, feelings and even actions, that the living can throw at the world. This is the classic way a demonic presence grows during the three stages of a diabolical haunting (infestation, oppression and possession).