So what does Absalom see happening here?
David is mad, but that’s all he is. He won’t do anything. He is an impotent leader and ruler. Unwilling or unable to carry out justice.
After all, he remembers that his dad committed two capital crimes, adultery & murder, each requiring death, but no punishment was carried out on him.
He figures that no punishment would happen to Amnon either. He was the king’s son, heir to the throne, “my dad won’t do anything to him either”.
So what happens? Nothing.