MIAMI — For someone whose daily work was to shed light on some of the darkest corners of the internet, Special Agent Daniel I. Alfin always managed to have a smile on his face, much to the bewilderment of his own colleagues. He was witty and tagged a heavy dose of sarcasm to the end of his sentences. He was smart and really good with computers, a skill he would pass on to many of his older colleagues throughout his postings in Albany, Quantico and Miami. By the time he was 36, he had become an FBI special agent in Miami investigating crimes against children, a milestone many law enforcement of...