The guy had some kind of obsession with this tribe for a reason nobody knows. If he was truly worried about 'Satan's Stronghold', then why not go to L.A. or San Francisco? Why travel thousands of miles into the middle of nowhere to harass a group of primitive aborigines who aren't hurting anybody? Why business is it of his how these people live or what they believe?
It was a real obsession. And it cost him his life. I believe he was not doing it for God or Jesus or for the people of the tribe. He was doing it for himself. He wanted the fame and notoriety that would come to him if he was successful. Likely he already had a book deal in mind.
No doubt he thought of these people as stupid primitives that he could easily manipulate. He chose poorly.