There's something otherworldy to me about the psychology of a violent criminal. To do violent crime is to make a lot of people very angry, and yet the majority of violent criminals seem not to run very far away as soon as they've committed a violent crime.
In some cases there has been recorded evidence of physiological differences (like a growth in the brain or a lack of development in some brain regions, reduced grey matter - as compared to non violent criminals) that have been found in some violent criminals.
