True crime has long been a popular niche in books and TV. Now it’s a mainstream obsession, thanks to new storytelling formats.The latest example is the 10-part Netflix series “Making a Murderer.” Before that there was the HBO series “The Jinx”... and before that the podcast “Serial.” All these works re-investigate old murder cases, sometimes turning up new evidence. This kind of journalism can be a public service. It’s also full of ethical pitfalls. What are they?