I can well imagine that after his first defeat, the European powers showed enough respect to a head of state not to execute him (or for other reasons) and therefore exile him to Elba.

But when he escaped, took over France again and was defeated at Waterloo, what was the reason for keeping him alive and in exile in St. Helena where he could have escaped yet again and tried for a THIRD time?

Heads of state had been executed before Napoleon (Charles I of England) so why was Napoleon spared?