The Trope
In comic books, TV shows, and movies that feature heroes/superheroes, the trope of the "no kill rule" runs rampant. So ubiquitous as to have its own page on TV Tropes, many fictional heroes refuse to cross the line from beating up the bad guys to ending a human life. In his more recent incarnations, Batman refuses to use a gun. Daredevil wants criminals to be brought to justice using the legal system. Superman took a vow to never kill. Even Jedis try to avoid killing when they can. There are times in all of these books/shows/movies when a judicious use of killing could be used to prevent the loss of many innocent lives. Often times, this is something that the hero is forced to grapple with at some point. As Peter Parker knows, "with great power, comes great responsibility."Not So Machiavellian
Machiavelli wrote in The Prince that rulers must act in accordance to a different moral code than a regular person. For an investigation of power, one could argue that Machiavelli would say the same about any person in a position of tremendous power--such as a superhero. While they don't rule countries or kingdoms like a prince, superheroes represent power in our culture today. They have abilities that no human has--extreme strength, the ability to fly, invisibility, endless money with which they can buy cool gadgets--and that is what makes them an embodiment of nearly unlimited power. With such power, Machiavelli would argue that these heroes need to occasionally take a human life in order to prevent that human (generally the villain) from perpetrating more evil. So what does it say about our society that most of the heroes in our books, shows, and movies are reluctant to kill, even if there would be benefits to it at certain times? Does it show that we value human life beyond all else and find it unforgivable for a human to take another human's life? But what if the human that the hero doesn't kill goes on to kill 50 other people? Fifty lives could be saved if the hero would choose to end just one. Even though the math seems simple, this is a hard thing for people to accept. In addition to being representations of power, superheroes are often supposed to be an ideal human of sorts--noble, self-sacrificing, and merciful. Just as we hate to see our superheroes kill, we hate the idea of our government torturing our enemies or killing civilians in times of war. While many people say that they are "Machiavellian" or agree with Machiavelli's principles, it says something about our ideals that our fictional heroes are not so Machiavellian.