Some people with the disorder are extremely thin-skinned and vengeful, said Mary Ellen O'Toole, a retired FBI profiler.

NPR: Court File: LA Ex-Cop Was Disturbed, Self-Obsessed

"We look at behavior and what's going on in the person's life, the social dynamics and what are the personality issues that make that person think acting out dangerously is a way to handle their problems, " said Mary Ellen O'Toole, a former FBI special agent and criminal profiler.

CNN: Gun debate: Where is the middle ground?