J.M. DeMatteis' new IDW series THE LIFE AND TIMES OF SAVIOR 28 has a superhero that wants to give up a life of violence. And Simon Williams does the same.
The Life and Times of Savior 28 is a superhero murder mystery where the Victim is Savior 28. The titular superhero, a Superman/Captain America pastiche (with a little of the Spirit tossed in) wants to give up his life of violence, saying it never really accomplished anything. J.M. DeMatteis said on the Word Balloon podcast that it's based on an idea he had for CAPTAIN AMERICA #300 back in the 1980s.
http://media.libsyn.com/media/wordballoon/WBjmdematties2009.mp3 There's similar concept in NEW AVENGERS #51, where Simon Williams, a.k.a. Wonder Man, said using violence to uphold justice has caused nothing but heartache and death. He's even sorry he kept getting "sucked back" into the Avengers.
http://comics.ign.com/dor/objects/734952/new-avengers/images/new-avengers-20090318044216533.html It's interesting that a superhero comic, even a "deconstruction" like Savior 28, would talk about violence not solving anything. Plenty of Superman and Batman stories have shown how Clark and Bruce fight crime in their "day jobs."
I'm trying to think of what kind of superhero would give up violence. The two I can think of are Batgirl and Spider-Man. Cassandra Cain was raised in violence, and her becoming non-violent could work. Peter Parker, a former wallflower, could get tired of beating people up and solving problems with his fists.