Try telling that to someone who has experienced what many call demonic activity.
This is my blog with my opinions, so that is exactly what you are about to read. I personally believe that there are indeed both good and bad ghosts, just like there are good and bad people. A subject that has come up in my mind periodically, and one that people bring up with me on a fairly regular basis, is whether or not a good ghost will try to frighten someone.
I don't think a good ghost will intentionally try to scare anyone. I have met people who at the slightest sign of paranormal activity become deathly frightened, like a small child who is afraid of a puppy the size of a grapefruit simply because it is a live animal. If you are such a person (be honest with yourself) then whether of not you are frightened is probably not a good gauge of a ghost's intent.
I have lived in multiple places I believe were haunted. From my own experience I have come to the conclusion that most good ghosts are curious but are cautious to not intrude too much. However, there are some rude ghosts (just like there are some rude humans) who are not malicious and just need to be told firmly to be respectful of the living.
And then there are the bad ghosts. I have had the misfortune of being around a few, and I can tell you that anyone who directly experiences their activity will be shaken sooner or later. If you are a tough person who does not scare easily (like me) then they will up the ante to frighten you. They get a kick out of making people uncomfortable. I personally believe they feed fear, heartache, fighting, violence and anger almost as if it were a drug. They engage in activities to stir up these emotions in the living who have the misfortune of living in "their" house. I can also tell you that such ghosts are not to be toyed with. If you feed them enough with negative feelings or behavior, or if you become aggressive with them, you can risk putting yourself in a very dangerous situation. I also highly advise against trying to communicate with these ghosts since they are manipulative and dangerous. Would you go have a nice talk with Charles Manson? I didn't think so.
So will a good ghost scare you? I don't see why one would do so intentionally. I have experienced and read up on many hauntings where evil ghosts (usually of the worst kind) will masquerade at first as something good, such as a spirit of a small child. But any good ghost, no matter who it claims to be, will not try to severely frighten the living or give poor advice (such as killing other people). Such ghosts need to be treated the same way you would a rude and manipulative person.