. >> before elliot rodger, there were a number of online misogynistic communities operating. really flips the switch from a community that is angry online, operating within the digital space, to a community that has the potential to carry out real acts of violence. >> reporter: his blind rage, targeting strangers, like promising student katie cooper. her parents left devastated. >> he attempted to get into one of the sorority houses with a tremendous amount of ammunition. instead, out of frustration, he got in his car, drove around the corner and saw my daughter. my daughter paid the price. so we like to believe that that wrong place-wrong time scenario helped save 50, 60 girls' lives. so, you know, kind of a silver lining. and, you know, when you're in our position, you look for these. you know, wherever you can. k just last year, two copycats. a van mowed down pedestrians, killing ten people, and then at a tallahassee yoga studio. both attackers allegedly posted online that they sympathized with roger. "nightline" is choosing not to show you more of roger's videos. but it's