He would have known what he was supposed to do and what he was supposed not to do. But his knowing didn't stop what happened to Naboth.
The Lord acted to give Ahab a reminder of what he should have already known. The Lord sent the prophet Elijah to go to the king. Tell him "I will bring disaster on you."
God's commands had been directed to ensure fairness and well-being for everyone. By violating the rights of Naboth, they have violated the commands of God. In their Preaching the Old Testament, Allen & Williamson ask
"Is there ever any limit to how much of the world's riches the wealthy can commandeer for themselves while others live on the street?"
Harming the weak is violating the commands of God, then and now.