"What Can Neuroscience Tell Us about Evil? Advanced brain-imaging techniques have begun to point to specific brain patterns common among sociopaths." The article also notes:

Of course, not everyone demonstrating these brain abnormalities ends up a killer. Some individuals with limbic underactivation end up in heroic professions, becoming firefighters, police officers, or fighter pilots, possibly because of a reduced fear response and a need for strong emotional stimuli. One theory is that other triggers, such as severe childhood abuse or neglect, are needed to turn people with already suppressed emotions into cold-blooded killers.

Plus there's also the issue of free will...