Napoleon did not 'destroy' Saxony. That objective was accomplished by the allies at the Congress of Vienna where Prussia demanded, and received, about half of Saxony, along with half of the Saxon army, and their king was imprisoned. They also were stripped of men and supplies for the allied war effort to a level never demanded of them by Napoleon. They would later mutiny against the Prussians and almost got hold of Blucher, who escaped the mutiny through his headquarters back door.

In 1813 Napoleon was urged by some of his subordinates to devastate parts of Saxony to hurt the allies, along the lines of what the British and Portuguese did in Portugal to Massena's army. Napoleon refused, as Saxony was an ally. The brutality belonged to the allies, not to Napoleon.

So much for Napoleon being 'a merciless tyrant.'