How did Amon die?

Answer: Killed by his servants

Amon's servants conspired against him and killed him in his palace; the people then made his son Josiah king.

“Then the servants of Amon conspired against him and killed the king in his palace. But the people of the land killed all those who had conspired against King Amon, and they made his son Josiah king in his place.”— 2 Kings 21:23-24 (BSB)