Child abuse
Some people have contended that the events in Columbine, Colorado were a direct response to emotional abuse by classmates (commonly termed "bullying" or "teasing").
In all cases of child abuse, it is understood by psychologists that the primary benefit realized by the perpetrator is psychological (ie, emotional). Frequently, the perpetrators were themselves abused as children. They then learned unhealthy ways of interacting with others, of exerting power (ability to influence others) and control (ability to deflect or redirect others' influence), and of disciplining children. This dynamic is responsible for the cycle of abuse.
It's important to understand that this is not simply a matter of habit but that victims of abuse may feel a powerful compulsion to relive the trauma they suffered. Some people, perhaps having deeper emotional reserves (or perhaps having none), will inflict the abuse on themselves or instigate situations to force an abuser to inflict it on them. Other people, seeking control over the abuse, will become perpetrators, inflicting the abuse they suffered on someone else. In this latter case, the perpetrator relives their trauma vicariously, by reversal with or projection into the victim.
See also