We see an immediate result of David’s sin. Just as he was unable to control his lust, neither was his son. Probably because David was also guilty of sexual sin, he does nothing to Amnon which causes more problems.
Sin has a way of multiplying. The ones we get away with tend to happen again and escalate. The ones where we get caught cause mistrust and broken relationships. The best answer is to avoid sin before it starts. Second best is confessing it, avoiding it in the future, and leaving it behind.