Why was Napoleon exiled and not executed?

Napoleon Bonaparte, thought to be one of the most influential and polarizing men in history, was, after his abdication in 1814 and again in 1815, exiled rather than executed. One might wonder why? On the surface the monarchies of Europe did have their reasons to do so. After all he was the man who sought to bring the British economy to its knees, implementing the so called Continental system, the man who on more than one occasion embarrassed the Austrian and especially the Prussian military, thought to be at the time the mightiest land army in the world, he was a person first admired by tsar Alexander but would later be branded as a tyrant and an enemy of the orthodox faith by the same man. Not the mention the countless lives lost, the destruction of Moscow as a consequence of Russia’s scorched earth policy, and the invasion of Spain who was at the time an ally of France.