John went out of his way to be vaccinated against a host of diseases. He also spent a week alone before going to the island and was thereby quarantined to a degree, though certainly i don't know how he interacted with the fisherman. he was aware of the potential dangers of bringing pathogens and took fairly extraordinary measures to avoid that, though clearly there is only so much you can do. his heart was to go and live among them, learn their language and culture and, in time, introduce them to Jesus. He didn't have any delusions of grandeur. He read over 100 books in the 21 months leading up to going, prior to this he took a special course in linguistics so he could know how to learn their language. he also spent years learning outdoor survival in various ways so that he would be prepared to live on an island without any modern conveniences.
Again, you don't have to agree with him, but to try to turn him into a genocidal maniac is so far beyond the pale it should embarrass you.
As for those quoting from Romans 13, I can't tell if you are serious, but that is a shockingly thin Biblical analysis. Paul himself regularly disobeyed the laws to not preach the gospel. Peter escaped prison in Acts 12....which I am fairly certain was against the law.