In his classic book “Killings,” Calvin Trillin said that writing about violence is best when it allows us to delve more into how people lived than into how they died. This claim has been tested over the last dozen years, though, given the unrelentingly gruesome news out of Mexico, a country where I lived for 10 years ending in 2004: mounds of headless bodies, corpses hanging from overpasses, a man known as “The Soupmaker” who dissolved cartel victims in acid baths, 43 students incinerated, etc.