A Clear Conscience......

What does it take to have a totally clear conscience? Does one have to confess all their wrong doings to everyone or can you just make peace within yourself with the things that haunt you? What of your criminal minds? Are they somehow immune to the feelings of a conscience and if so how does one override the teachings of the world and retrain their mind to not feel any remorse for doing something that is wrong. I wonder if even the people with the blackest hearts ever feel that small tinge of guilt while they are alone and their mind is thinking.
Is a conscience something that is instinctive or must it be taught to a person. And if it is instinctive how do some people in life go around without a conscience? Have they strayed from what we think of as normal human behavior and adopted their own set of rules in life or where they just born with some kind of defect.........