A day after the Mexican army killed the country’s most powerful drug lord, the picturesque town where it happened was a study in contrasts. Tourist shops in Tapalpa were open Monday, and workers were on the job. But gunshots also rang out, and in the street was a dead man lying beside a bullet-pocked vehicle. Meanwhile, heavily armed Mexican security forces kept up their battle with cartel gunmen following the killing that sparked a surge in violence and put the country on edge. Cartel fighters...