First he goes into exile on the island of Elba. Why was he allowed to do that? Then he escapes, and takes back the power for 100 days. Then he loses again and is confined at the island of Saint Helena. Why wasn't he executed?
http://www.reddit.com/r/AskHistorians/comments/1ra1oe/why_was_napoleon_exiled_and_not_executed/
To cut a long story short the execution of heads of state was taboo and they didn't want to make it common. After all you don't want it make it common when you yourself could one day suffer the same fate. It was one of the reasons why the execution of King Louis during the French revolution was so shocking.