Sociological studies have proven there are no absolutes when it comes to deviance. Words like "always" and "never" don't really apply to Human behavior, so "common sense notions" like 'once a criminal always a criminal' or 'bad people do bad things' are a little short-sighted and lack a depth of understanding.
Ted Bundy was a serial killer who murdered dozens of women. He also once worked on a Suicide Hotline, saving lives. He may have been an evil person, but there was still some good in there somewhere. It was eventually overwhelmed by the evil but it was still a part of him. Bad people can do bad things, but that doesn't necessarily mean they ALWAYS do bad things or that EVERYTHING they do is bad.
The converse is true when dealing with "good" people. Many people who were basically "good" have been known to do some very bad things. The Christians, who you would normally think of as good people, undertook the Crusades, repeatedly raping, murdering and pillaging their way across three continents, all to enrich the Church and broaden its power. They also perpetrated the Salem Witch Trials, The Spanish Inquisition, murders of scientists and visionaries of their Age who were condemned as "Heretics" for saying horrible things like "the world is round." Countless episodes of "Conversion by the Sword" perpetrated upon "backward Pagan savages" like the Maya, the Inca, the African tribe people, and the American Indians, to name but a few. There are good people, but that doesn't mean that everything they do is always good.
The truth is, as Humans, we run the gamut. No one is purely good or evil. We run the spectrum, with most of us falling somewhere near the middle. Sociology and countless studies of Human behavior and interaction have borne this out, time and time again: "Common sense notions" really have little place in the study of a People so diverse in their attitudes, beliefs and conduct.