Why don't murdered people exact revenge on their murderers?

Don't tell me dead people can't hurt others; here is evidence that they can. The video author states that this is genuine, un-edited footage taken from a security camera: 

A year ago, an 18-year old teenage male was murdered in his car by a group of people he assumed to be his friends. They inexplicably turned on him in unison and killed him right there in his car in the middle of the night.

He also had a girlfriend. This occurred in quite a small town, so if the murderers were being beaten by some ghostly kid after this I'm sure it would have been reported. what kind of person would not say anything to his girlfriend, or go on the news and tell people who his murderers were at least.

(The victim in this case died of strangulation. Perhaps he may have been left with the inability to talk, but if ghosts can interact with matter directly as seen in the video, then he would have been able to write messages and send them to authorities, or exact revenge on his traitorous murderers.)