On the whole, when you think of superheroes, you think of them as beings that don't kill. While famous figures like Batman and Superman aren't against beating a bad guy up, they are against murdering him in cold blood. (Unless you're a cosmic evil, but then the stakes are a little higher, so that's maybe fair.)
But, as it turns out, plenty of morally upstanding superheroes have killed a surprising amount of people. Even folks with supposed rules against killing aren't exempt, against all logical expectations.
This said, there are also plenty of heroes who have no such rules, and who have merely killed a crazy amount of people, for reasons that range between being totally justifiable to entirely based on a whim. These characters are often used to remind us of how thin the line between being a hero and being a villain can be. It shows heroes at their worst, yes, but it also shows us the willpower it takes to see someone do awful things and lawfully put them away.
Every hero has a couple skeletons in their closet - but as it turns out, this goes for some heroes significantly more than others.