From the top of my head (not having read the story, jsut this blurb) :
1. The island was a research facility of somekind. Military grade weapons, inhumane scientific research (Ilse of Doctor morau and City of lost children type)
2. The man did something between the time he got on the island and the time he gets sent to his cell. He just doesn't remember. Maybe exoplore this "lsot time" trough journals, records, second hand accounting. Make an inquiry into it?
3. An island jsut blew up. Someone -else- is going to get there just because people are curious. Did any items survive (see 1 and 2 above)
"To absent friends, lost loves, old gods, and the season of mists; and may each and every one of us always give the devil his due."