From deep under the city comes a lesson in thinking things through: When a passenger on an uptown 6 train tried to break up a couple’s nasty-sounding fight, the man punched him in the face. So the would-be good Samaritan got out his pepper spray and blasted the guy. This did not go well. The spray filled the car and sent three people to the hospital, while the couple slipped out in the confusion before cops could arrive. Alas, there’s just no established etiquette for using pepper spray on the subway.