This assumption may also arises from my own personal experiences with others who dress in a similar fashion. I begin to wonder whether they enjoy gory movies, piercings and listening to Green Day. These are often characteristics of people who dress in this fashion.
In relations to Columbine, the TCM (Trench Coat Mafia) were known to be a group of anti-social goths. Because of media's belief of the relations to Dylan and Eric it brought attention to the subculture. Teens who often dress in the fashion of the TCM are often referred to as problem/anti-social children or potential school shooters.
In conclusion, the way someone dresses can cause people (like myself) to create assumptions on ones lifestyle and traits, but it is always important to get to know individuals, before judging them