I have a good friend who was plagued for about three years by a spirit that appeared in his room. He usually would be awakened by a horrible feeling like he was being watched. When he looked up, he would see a man in form. He sometimes appeared wearing a hat and other times he held it in his hands. He never uttered a word but would stand there and stare, sometimes smiling. It wasn't a warm smile.
His room mate saw the spirit once and from then on slept with the light on the rest of the night and then found another room mate. After about three years, the spirit just stopped appearing. Max says he afterwards felt watched at times, but even that stopped.
There are evil spirits that are unembodied, which is what Satan and his angels are. There are others that are disembodied spirits, people who lived on the earth but either fear damnation if they move on or who just don't want to move on. These spirits are not able to take a person's life, though they can make people miserable. They also, through the power of suggestion, try to entice people to murder.
The spirit that appeared to my friend didn't seem to be tied to a place, like disembodied spirits are, but followed him from place to place. This spirit didn't appear often, but it happened enough that it troubled him. He didn't tell me until I had known him for some time. I'd stake my life on his word.