The pirate on the island is seeking riches, but there are king's officers that are hunting you. Collect as much gold as you can and kill your enemies before they will.
The map which you bought from the strange salesman had a mark on this island, therefore it should be full of gold that were lost a long time ago. You can become rich if you find it, so discover the island and see where it could be hidden. However, seems like you are not alone on this island. Be careful as there are king's troops hunting pirates.