We'd like to omit the Littleton duo from this list, if only to deny them another smidgen of the notoriety they desperately sought. But there's no denying that the Eric Harris and Dylan Klebold's rampage at Columbine High School on April 20, 1999 was among the century's most infamous crimes. The duo — Harris an extroverted, apparently sociopathic army brat, Klebold a depressed loner — marched into their school and killed 13 people before turning their weapons on themselves. It wasn't the first school shooting, and it wasn't the deadliest (that odious designation belongs to Virginia Tech killer Cho Seung-Hui). But Columbine was a shocking, senseless tragedy which left a scar that, 10 years later, shows no signs of fading from our consciousness.