Does Karma exist? Can we make up for bad behavior?
The human mind is not very rational. It's driven by emotion, delusion, and devotion to a great extent.
Therefore, we are not entirely responsible for our behavior. That's why when I hear a criminal talking about the crimes he committed I don't believe the person is scum but mostly misguided and lost. Personal responsibility is a big factor but only that.

Environment shapes a person's mindset and behavior. Young people on average are not as responsible as adults as a result of different hormone levels, lack of life experience, societal expectations, or whatever. So how responsible is an individual with an abusive upbringing for belligerent or even violent behavior in his teens and twenties, and can these individuals compensate by acting ethically for the rest of their lives? — Steve555

Seems like you're asking two questions.

1) Can we make up for bad behavior?

There have been discussions on the forum before about asking forgiveness. My position has always been that apologizing is not the way to deal with our own hurtful behavior. At least it's not enough. To me, you have to 1) stop doing it 2) acknowledge the error 3) make it right to the extent you can.

2) Does it make sense to hold people fully responsible for their actions?

Yes, definitely. Socially and psychologically we have to. Holding someone responsible is not the same as judging or blaming, which I think are useless, although some people get satisfaction. Holding them responsible means just what I listed above. They should stop, acknowledge, fix and if they won't, we should do what we can to see they do.

Of course there are mitigating factors - age, history of abuse, mental illness - but not holding someone responsible is disrespectful to them.

well philosophically speaking, you shouldn't feel bad about the past

The human mind is not very rational. It's driven by emotion, delusion, and devotion to a great extent. — Steve555

It is true that the human mind is not very rational. But rationality is still one aspect of the mind.

Therefore, we are not entirely responsible for our behavior. — Steve555

How responsible are you? 86%? 23%? 50%?

If you become angry and kill somebody, who--besides yourself--could be responsible? You probably want to be held responsible for the good things you do, right? Well, if you can be held responsible for the good things you do, then you can also be held responsible for the bad things you do.

If you have done bad things to other people (let's say they were merely hurtful acts which were not actual crimes where the state intervened) is there anything you can do about it? Yes, there is. In a nut shell, a) repair the damage you did to other people (as well as you can) and b) go and sin no more.