I am not going to pass judgement on this man. For all I know he may have been right to do what he did. He seems to have understood the chance he was taking.
Yes, he no doubt did his research on these people and he had to know in advance that it was illegal for him to visit the island and that the people living there have been known to kill outsiders.
According to this young man's letters, he went to the island the day before he was killed and offered gifts. In return he was met by people yelling at him and shooting arrows at him. God allowed him to safely escape and preserved his life that day, but it was made clear by the actions of the islands inhabitants that he was not welcome on their island and they were willing to kill him.
By returning the following day, he clearly showed a lack of good sense and judgement.
This young man also violated the law by attempting to reach the island and caused those whom he paid to take him close enough to the island to reach it to also break the law. This was clearly wrong on his part.
In addition, he didn't consider the welfare of the people he was trying to reach. They have had very limited contact with the outside world and they likely have no immunity to many diseases. His visit potentially put this entire tribe at risk for disease which could wipe out the entire population.
And lastly, his death has brought unwanted and negative attention to these people.
Was it God's plan or this young man's plan to take the gospel to these people? If anything it appears to me that his actions were selfish. He ignored the law, ignored the wishes of these people to be left alone, and he has now unnecessarily lost his life leaving his friends and family in a state of mourning.
Loving others means respecting them. Following Christ requires us to respect authority. This young man's actions are a good example of how not to do missionary work. I hope his death will not be in vain and will serve as a lesson to others who are considering the mission field.
