What have we learned since Columbine?

11 years ago, the word "Columbine" broke into our collective conscious. Amidst tragedy, chaos, and suffering, psychologists have sought to make sense of this and other school shootings.
I'll never forget April 20, 1999. I was in college. My older sister was living in Colorado and, on that day, was visiting a school close to Columbine High School. At the time, she was teaching at a charter school, having recently earned her degree in elementary education. On a break from my class that day, I learned about the tragedy. My first thought went to my sister. Was she okay? Were there gunmen at other schools? I panicked. Luckily, she wasn't anywhere close to Columbine High School that day. But that fear haunted me, ultimately driving me to understand why people commit such violent acts.
Scientists can offer an educated guess regarding why events such as Columbine happen. There are dozens, if not hundreds, of factors that increase the likelihood that someone will behave violently. Psychologists call these "risk factors." It's beyond the scope of this post to discuss all of the potential risk factors for school shootings. Instead, I'll focus on two.
First, the Columbine Shooters--Eric Harris and Dylan Klebold--experienced acute and chronic rejection. Rejection, like physical pain, hurts. Is the pain of rejection enough to cause people to behave aggressively? In the years since the Columbine shooting, my colleagues and I have shown that rejection causes people to behave aggressively. Rejected people blast others with loud and prolonged noise, dole out large amounts of hot sauce to people who hate spicy foods, and thwart others' opportunities to obtain desirable jobs. Social rejection causes people to behave aggressively even toward innocent third parties, suggesting that victims of rejection-related violence need not be involved in the rejection experience.
Second, the Columbine shooters showed signs of having narcissistic personalities. In their home videos made prior to the attack, they declared that the attention they would gain from their violent actions would finally give them the respect they deserved. Narcissists have fragile and inflated egos. When narcissists feel threatened, look out. Indeed, Jean Twenge and Keith Campbell showed that narcissists behave very aggressively when they've experienced rejection.
At the end of the day, no one will ever have definitive evidence telling us what is necessary and sufficient to cause people to commit heinous actions such as the Columbine shooting. Feeling rejected and having a narcissistic personality may increase the likelihood that people will commit such violent actions. But there are many other factors at play. In the meantime, we'll keep doing our best to uncover the mysteries of why people engage in such acts of violence.