Of necessity I will address this as a Christian, since that is what I am.
I have been aware of the Sentinelese for some years and read about them with interest. The ban on contact seemed prudent, imo, both for their own possible vulnerability to disease and protection of outsiders.
When I read about this young man's death, I was more than a little conflicted.
On the one hand, as a believer myself, I had to respect his conviction. He apparently was firmly convinced that he had a calling from God to bring the Word to these people, even knowing it could easily cost him his life, and was willing to take that risk. That is a level of faith I respect.
On the other hand, his actions were, from the viewpoint of prudence, reckless in the extreme. He knew their usual reaction to outsiders. He had to know he would probably be killed.
I haven't yet seen any indication that he was aware of the viral contamination risk, the possibility that he could be carrying microbes that they had no immunity against; that his visit, if successful, might wipe out the people he hoped to enlighten. If he was unaware, that is a terrible ignorance that a little study could have cured, and no excuse for it. If he was aware of this and went ahead anyway, I am staggered by the hubris, overconfidence or lack of prudence involved. I'd really like to know what his thinking was on this matter, as it is extremely important (or should be) for anyone with benevolent intentions regarding contact.
He doesn't seem to have done his homework. A few people have had non-hostile limited contact with the Sentinelese by staying in the surf and offering gifts, while wearing no or minimal clothing. The Sentinelese seem to react badly to uniforms or full clothing, possibly because of how different it makes people look from them, or perhaps oral tradition from previous contact with soldiers. This has been noted in some of the literature available about them. *I* have read of this, from way back here in the USA with no intentions of going anywhere near the Sentinelese... why didn't he know this??
Many unanswered questions that do not reflect well on his level of preparation and planning. How did he intend to get past the language barrier? No one even knows the lingo, or whether it is related to any other known languages. Was he a linguist? Not that I have heard.
From what I have read, his first encounter did not go well. A boy of perhaps 10 shot an arrow at him, which did not injure him but did pierce his Bible. He survived only by fleeing back to the boat.
Personally, I would have taken this as a sign, and not an encouraging one.
He persisted and got killed on his second attempt at contact.
Secular reason says this was a predictable and probable outcome, resulting from ignorance, poor preparation, or just plain recklessness.
From a Biblical perspective: While martyrdom is honorable, I see no scriptural compulsion to seek it out in this manner. The Apostles were told, when they came to a city that would not receive them or their message, to depart and "shake the dust of the city from their sandals". This and other scripture suggest to me that trying to impose the Gospel on those who vehemently want no part of it is literally fruitless and not recommended.
He believed he had a calling from God to do this. It is not my place to question another person's conviction in such a manner... but in retrospect, I'm afraid I have to question it. I don't think God throws away His good servants in this manner, where it will do no good. One can think one has a certain calling, and be mistaken.
I think God gave us brains and the capacity to reason for a purpose, and that we are to use them. I think this young man went to his death because he did not.
That is merely my opinion. I have heard other fellow believers say that we don't know if his death may serve other important purposes, as a witness and so forth. Perhaps they are correct; personally I see little benefit given that the common reaction in the secular world is that he was stupid and asked for it.
Personally, I admire the courage of his convictions and believe he meant well, but am appalled at his lack of prudence and apparent failure to study the situation adequately before making his attempt.
I wouldn't go within 200 yards of the island without a much better plan for how to make peaceful contact and develop meaningful communications... and preferably some sort of official authorization.
Edit: oh, and as for prosecuting a native for his murder, that is nonsense for obvious reasons.