Levin speculated that Martorano might be more like landlady Dorothea Helen Puente, who in the 1980s killed for profit. She ran a boarding house in Sacramento, Calif., and cashed the Social Security checks of nine elderly and mentally disabled boarders.
Martorano rationalized his murders as acts to protect family and friends, even citing the values that stemmed from the "troubles" between factions in Northern Ireland.
"Most people in Northern Ireland never killed anyone," said Levin. "I have been there twice and they are a gentle, decent people and not serial killers."
Bulger's defense attorney, J.W. Carney, called Martorano a "criminal psychopath."
"He would kill people almost randomly," Carney said. "He would kill people as easily as we would order a cup of coffee. ... The federal government was so desperate to have John Martorano testify ... they basically put their hands up in the air and said, 'Take anything you want.'"
Experts consulted by ABCNews.com agreed that Martorano and other gangland criminals could suffer from a personality disorder, lacking conscience and empathy.
"We had a lot of problems with people," Martorano testified. "And you know, you just killed them before they kill you. It's kill or get killed, at times."
Kenneth V. Lanning, a retired special agent at the Behavioral Science Unit at the FBI's National Center for the Analysis of Violent Crime, said there is a difference between being psychotic, which is a mental illness, and being a psychopath, which is considered a legally culpable disorder.
"Lay people and the media constantly confuse and interchange the terms psychotic and psychopath, and occasionally throw in psycho to make it worse," he said. "Schizophrenia is an example of illness that causes psychosis. By definition, those who are psychotic have hallucinations, delusions and disordered thinking, or a combination of these."
Psychosis sometimes reduces a person's responsibility for a crime with court findings of "insanity," "diminished capacity" or "guilty but mentally ill," according to Lanning.
But being a psychopath involves a personality disorder, he said.
"It is not something you have, it is what you are," said Lanning. "Psychopaths know others consider what they are doing wrong. They just don't care."
Psychopaths, often excellent liars and con arts, are also held responsible for their criminal behavior, according to Lanning.
"If they were not held responsible for their criminal behavior," Lanning said, "there would not be many serious criminals left in prison.
"As they lie to you, they can look you right in the eye, swear on a stack of Bibles, and not blink an eye," he said. "They lie for the same reasons everyone lies -- the truth is damaging or embarrassing. They also lie for reasons that most people do not -- just to see if they can get away with it and to play with people."