SAN DIEGO — Even as journalists in dozens of cities across Mexico held vigils and demonstrations in recent weeks to protest vicious killings of two of their colleagues, the threats against local Tijuana reporters continued to pour in on social media and in person. “I know where this reporter lives. I know where his whole family lives,” posted one Facebook commenter on a screengrab from a video from the scene of photojournalist Margarito Martínez Esquivel’s Jan. 17 murder. The comment was about one of the reporters who had rushed to cover the fatal shooting of his colleague. The siege of violen...