Edited sentence: Isolation seems to be the most common factor affecting school shooters, but since Eric Harris and Dylan Klebold, both perpetrators of the Columbine high school massacre, had at the same instant killed over a dozen people, psychologists had lead to re-thinking how a school shooter thought.The chilling effect is that the perpetrator feels it is justifiable to kill, maim or wound others as a response to perceived intimidation, exclusion from social groups, or physical and/or psychological bullying.The perpetrator feels frustration, not being able to express him/herself in ways that would release stress, such as telling their problems to friends