I can understand Absalom's anger when Amnon raped his sister Tamar and David did nothing to punish Amnon.* Amnon was definitely a low life.
But Absalom made a fatal error. He decided his father's negligence gave him the right to play God.
He not only violated the Sixth Commandment by murdering his brother, he violated the Fifth Commandment by dishonoring his father.
All parents are sinners and they will be held accountable for their sins, but we never have an excuse to dishonor them or treat them badly.
Nor can we blame them for our adult choices and sins.
Years ago, a parent had to be genuinely abusive to be despised, neglected, or sidelined by their adult children. Now, it's become popular for adult children to judge, reject, and neglect good parents who made mistakes.
We've become a nation of Absaloms, whining about our parents' failures and overlooking our own.
It's popular culturally, but it violates God's commands.
In the end, David enjoyed God's favor and praise. Absalom did not.
* David's refusal to act could have been based on his guilt over his own sexual sin (of which he'd repented) or because the law required a witness to such a crime and there were none, or simply because he refused to treat the crime as it deserved. Scripture does not explain.
To read the story of Amnon and Tamar: 2 Samuel 13
To read the story of Absalom's dishonoring of David: 2 Samuel 15-18