Before he was killed by an isolated tribe on a remote Indian Ocean island, John Allen Chau, a young American on a self-propelled mission to spread Christianity, revealed two things: that he was willing to die, and that he was scared.

“You guys might think I’m crazy in all this,” he wrote in a last letter to his parents. “But I think it’s worth it to declare Jesus to these people.”

Part letter, part journal, in 13 pages with many cross-outs and messy scrawl, Mr. Chau laid out a disturbing account of his final days on North Sentinel Island in the Andaman Sea east of India.

He tried to give gifts. A boy shot an arrow at him. He expressed fear, fatalism, frustration and some humor.