In fact, although a person's snap into violence may come as a total surprise, in most cases there is a psychological buildup to that point, said Dr. Peter Ash, director of the Psychiatry and Law Service at Emory University in Atlanta, Georgia.
"There's a pathway to violence that starts with some thinking and then fantasizing about a plan," he said. "There may be a more explicit planning phase that other people don't particularly notice."
The fantasy of killing others may turn into intention, leading the person to track victims and obtain weapons, Ash said.
The psychological buildup to a violent outburst with the intent to kill usually takes a minimum of a few days, said Dr. Lyle Rossiter, a forensic psychiatrist in Saint Charles, Illinois. However, in highly unusual cases, a person with bipolar disorder could experience a buildup of only hours, he said.
A person who has already decided to kill someone else may develop an "eerie composure," firmly believing that the moment to turn back has passed, said Dr. Charles Raison, a psychiatrist and director of the Mind/Body Institute at Emory University.
Despite the planning during the buildup, experts have found that a perpetrator often cannot recall particulars about the moment of the attack. "You would think they would give it a lot of thought, but sometimes they go into a somewhat dissociated state where their feelings are really kind of split off from what they're doing," Ash said. "They may even experience it as if they went on autopilot."
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Life experiences can also contribute to snapping. When mental health professionals evaluate perpetrators of violent crimes, they look at relevant defining events and personality traits, he said. For example, the person may have experienced or witnessed violence or abuse early in life, Segal said.
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Other warning signs include feelings of being hopeless, ashamed and trapped, Raison said. People with these feelings may announce ahead of time that they are going to kill someone else or themselves, which increases the likelihood that they will follow through with violence.
When people who are not psychotic are committing a homicide, some dehumanize or blind themselves to the person they're shooting, Ash said.
"It's striking when you talk to people who have done things like this, how they're really preoccupied with their own feeling and have in their mind stopped thinking of the other person as real full human being," he said.
