malchut royalty - Why doesn't Ahab have the right to just seize land as he see fits?

Ahab wanted a land from Nabot. Nabot doesn't want to sell. Well, Ahab is a king.

So Ahab frames Nabot for treason, kills him, and takes his land. G-d gets upset and curses Ahab and his descendants.

If Saul can kill the high priests and is still a tzadik, why can't Ahab?