/u/Searocksandtrees compiled previous links at https://www.reddit.com/r/AskHistorians/comments/8hixcb/why_didnt_they_execute_napoleon/ The first hit, by /u/kieslowskifan , does touch on distant versus nearby exile too.

But this is just a starting point, not to discourage further discussion. In particular, I hope someone can discuss the imprisonment idea, what was done with Louis XVII and Napoleon II.