When I was younger I used to believe people were good and bad but I've become more mature. Mature enough now that I think environment is the main factor which shapes how people behave and that in most cases people aren't "bad", but the behaviors are.

If a person has a healthy brain which functions normally, if a person has relatively normal intelligence (including emotional intelligence), then it's a matter of what did this person experience in their life in order for their brain to wire in a certain way so as to adopt certain patterns of behavior?

I think while some people are born with certain psychological or neurological deficits, this is not the case most of the time. The people born with the deficits are not "bad" or "evil", but simply are not wired for dealing with moral problems. Everyone has their unique brain wiring and all of us have stuff we are more wired for than others or less wired for than others.

Bad character traits can change over time. People aren't generally set in stone. You can be born with a brain wired one way and over time do stuff which rewires your brain to function in another way. I know because I'm an example, as I didn't always have the traits I have today and I develop new skills, traits, and characteristics over time.

People have to want to improve and that is critical. If a person decides they are as good as they are going to be then they limit themselves to being whatever kind of person they are today. If a person decides not to be satisfied with how they are today and to always try to improve no matter how small the improvements are, then over time a so called "bad person" can become a "good person" in the eyes of others. Unfortunately our society isn't very forgiving and doesn't really promote this way of doing things.