My response to such a question, since the writer is not a Christian, was as follows:
As a Christian, I know you know that we would consider the sin nature of the person to be why one rebels from that which is good. But, as you said, this does not mean “good” people are always good nor does it mean “bad” people are always bad. It appears that some people are born with more desire to rebel compared to other people, for I see this in my own kids. There is also reason to believe if some children only receive attention (although negative attention) because they do something wrong, they will continue to do wrong things just to get any attention at all. Sometimes it seems it could be that people “stay bad” simple because they have been stereotyped as “bad”, everyone talks, acts, and treats them like they are bad, so they continue to act as people see them as. I believe everything plays a part, such as personality, upbringing, choices one makes, but the battle is truly inside the person, and although I think we may can help them in some ways, the ultimate choice is that they have to turn their backs on what is wrong and decide to do right simply because it is right. Sometimes we would like to turn our backs on them, other times we may want to shake sense into them, but ultimately all we can do is hope something inside of them changes. (As for a Christian, we hope for salvation.)
To expound a little more here, we should realize that what man sees as “good” persons and “bad” persons may not be in line with what God sees as righteous and wicked persons. Scripture gives great example to the total depravity of mankind, but at the same time shows that man can do both outwardly good and evil things.
Some important things to note concerning “good and evil” are that  although man now has knowledge of “good and evil” that does not mean we have the ability to do that which is good, for Christ said, “there is none good but one, that is, God”,  although man may perform outward “good” acts of kindness, charity, etc., “man looketh on the outward appearance, but the LORD looketh on the heart” and “The heart is deceitful above all things, and desperately wicked: who can know it”, so where we may outwardly see an act as good, the inward look at the heart can be seen by God as selfish, prideful, and totally wicked.
Man is wicked because of his sin nature and desire to sin and rebel against God, yet a man can only be righteous by the repenting of his sin (to turn away from) and putting faith in the work of Jesus Christ and having His righteousness upon us. Even as Christians, we must battle the flesh and desires to sin, for God works within us to do that which is good and right.
So a lost person can be good and a saved person can be bad, but a lost person cannot be righteous nor is a saved person righteous by his own accord. Being good or bad does not determine heaven or hell for the person, faith or unbelief in the gospel of Jesus Christ, whereby one may repent of sin and put faith in the work of Christ for His righteousness, does.