Did you try approaching with each character individually, to see if the hostile reaction was just with one of them?
You can try hiring a henchman, and see if they can approach.
Did you speak to him earlier, using a dark mirror? If so, did you pass or fail the persuasion in the dialog? If you pass, he should give you a quest to kill Alexandar.
Some dark mirrors can be found on Black Ring corpses on the island, or you can speak to the group near the entrance to the elven temple (blocked by the shriekers).
