It's called Absolution and is about a superhero working directly with and/or for the police. In a world where superhero are as tied by regulation as regular cops.
At one point, the hero snaps, having seen too much evil, that he couldn't just take down. And he starts killing perps rather than bringing them in. Now I'm not usually a big fan of superheroes who kill. But I'm willing to make an exception for this one.
The comic is published by Avatar and has a great supporting cast and a really interesting main character. And unlike books like Punisher, Absolution recognizes that what the main char is doing is wrong. Even if the people he kills totally and utterly deserve what they get.
Some pages of it were posted here