Movie previews are an art form in and of themselves. To be able to show enough to entice the average viewer and tell what the movie will be about but not give away the plot. Music helps with a vital component as well. Will the be funny or serious? Scary or an adventure?
With the right edit and the right score for a movie, a rather benign comedy can give the appearance a rather dark and disturbed looking drama.
Moe Lane has some more thoughts about Ferris Bueller and Tyler Durden that may be on the disturbing side.