The Sentinelese are thought to be the last pre-Neolithic tribe in the world and have been hostile to outside visitors, including killing fishermen who washed ashore and shooting arrows at a helicopter during a welfare check after the 2004 Indian Ocean tsunami.
The Indian Government prohibits approaching within 3 nautical miles of the island.
When a young boy tried to hit him with an arrow on his first day on the island, Mr Chau swam back to the fishing boat he had arranged to wait for him offshore. The arrow, he wrote, hit a Bible he was carrying.
Here's a good summary I was just reading: https://www.abc.net.au/news/2018-11-26/police-struggling-to-remove-body-from-north-sentinel-island/10553704