Good questions. I suppose some ghosts may be considered evil because they are more negative energy than positive. But this does not necessarily mean they want to harm someone in the physical form. It simply means they are more likely to attempt to generate fear. I have heard stories of ghosts harming people in the physical, but these are stories without any investigation to back them up so it is difficult to say if they are real or not. I have seen no evidence of a ghost actually harming someone in the physical. That is not to say there have not been claims of such, but the evidence has not been presented to be convincing. With all the ghost sightings and paranormal events that take place in the world every day I'm sure there is something out there that would support this belief that a ghost could be negative enough to harm an individual - I just haven't found it.

Upon reflection, there has been one thing that might convince me a ghost can harm an individual in the physical form. Some years ago the television program Sightings did an investigation of a home in the "Heartland" or central United States. In this investigation, a single family home was being haunted. The events that took place, at first, were minor at first. But, as time went on, these physical events took on a more sinister action wherein the man in the house was being attacked physically. These physical attacks took the form of severe scratches on the man's arms, chest and back. If I recall correctly their child was also experiencing scratches as well. The family finally moved from the house. Peter James, a fairly well known psychic, went through the house and concluded that the person scratching these family members was a little girl who had died from some sort of illness that caused a severe high fever. 

Hope this answers your questions! Thanks for writing Eeeek!