What everyone else said. I am not too up on my NYC CN knowledge, but I know in DEAL WITH THE DEVIL, it was stated that Vic Orena was made solely on his ability to earn and that he never killed anyone. I don't know the veracity of that claim though.
I'm sure that there are plenty of made men with an aversion to violence. Especially if you came up in a Catholic household. I think most people have that inherently, unless you came up hard. Killing someone isn't easy. It's something you have to live with for the rest of your life, with consequences that may be spiritual in nature. Some people develop a thirst for it, which may or may not have been there in the first place, and those are the guys you are reading about. No one is writing books about the "gentle extortionist" or the "benevolent bookie."