If you ignore Gareth at the end (who is the most unpleasant person in this game so far IMO) and kill the Bishop, the worm and so on you can leave the Island.
If you pop back up the stairs though the guy just keeps asking if you are ready for battle and who you would like to help you. No-one comes to help (as there is no-one to fight).
There should be a dialogue option "it is OK I've already killed them all" or some such. Perhaps they should all just run away for the boat as that is what they normally do.
