I agree that many of them are psychopaths or border line psychopaths with severe antisocial tendencies. You can find these types throughout our society though, not only within the criminal element. Studies have shown that many CEO´s and successful businesmen for example, also possess these kind of personality features. They are often described as "men (and women for that matter) with ambition". The saying that "some people can literally step over dead bodies" may be more true than we are willing to admit. However, the difference between "respectable" people in our society, the ones we call "ambitious people" and a vast majority of criminals (including Mafia members) is that criminals suffers from learning difficulties and are very often school drop outs. Back in the day, many criminals only had three, four or five years of education which of course limited their chances of making it in the legit world.
A vast majority of criminals (I´d say 90 percent of them) also suffers (in one degree or another) from attention deficit hyperactivity disorder (ADHD) which is a problem with inattentiveness, over-activity, impulsivity, or a combination of these things. This is a mental disorder which is very common in our society. Note that I´m not saying that people who suffers from this are criminals, but a vast majority of criminals (studies have shown this) suffers from ADHD.