6. The Columbine Massacre
Since April 1999, school shootings have, unfortunately, become an almost yearly occurrence, but regardless of how many like-minded tragedies take place, no one will soon forget what happened at Colorado's Columbine High School. That's when two students, Dylan Klebold and Eric Harris, clad in trench-coats, casually walked into the building, killed 12 schoolmates and one teacher, and injured 21 others, before committing suicide once their murder spree ended.
Initially, Klebold and Harris planted bombs in the school's cafeteria, but those failed to explode. Thus, they grabbed semi-automatic guns, walked through the hallways and popped into classrooms, and opened fire on anyone in sight.
Various theories have come up as to why Klebold and Harris committed the act, ranging from the negative influences of video games like Doom and Wolfenstein 3D to bullying and the overuse of anti-depressants. Whatever the case, though, no explanations will ever alleviate the pain felt by the victims' families and friends.