A great hero is selfless; they are willing to sacrifice themselves for the greater good. This concept has been quintessential to the comic book genre, with perpetually every character at some point facing a scenario where they must at least show that they are willing to die so that others may live. So many heroes we thought infallible have died; just look at Marvels recent mini-series The Death of Wolverine to show how someone who is basically immortal can be so easily killed. What makes a sacrifice different to a death is that selflessness. The fact that the character knows that they are going to die, but they continue to fight anyway. It is the singular characteristic which makes a character a hero. Think of Agent Coulson in The Avengers; he knew he was not going to be able to stop the onslaught of Loki, but he died (at least temporarily) trying to save people. Comic books are a modern mythology; the ancients worshiped gods who were at the peak of human physique, and who would fight and die in glorious battle. These ideas have been translated into comic books now, where these glorious battles and selfless sacrifices have become an iconic part of the genre; just count how many of the following panels have become iconic.