First off, I believe that most of the world's problems are rooted in poor education and the inability to educate oneself. I don't think Columbine was terrorism and was just a mass murder. I think terrorism requires political or religious goals according to its definition. And the radical nature of terrorism makes it lack the possibility of treaties which were a prior historical norm.
I don't think the massacre was related to the Gothic subculture. But by using the words goth and gothic, speakers in the media made a link between dark aesthetic and school shootings; a connotation which probably continues with other generalizations in educational institutions between the scraps of actual learning. I think an inability to change one's opinion based on evidence and logic, and inability to analyze one's thinking, are both aligned with cultural conservatism (phrase has nothing to do with politics) which serves poor education.
tl:dr: Columbine was not a terrorist act. Cultural acceptance requires learning revolution.