I noticed it, too, and I thought it was a good touch, as it gave a little credibility to the fight. When a couple of thugs come at you, maybe you can incapacitate them without hurting them too much. When fifteen of them come at you, you just have to do what it need to be done if you want to survive the encounter, even if you have the power of kick-assery. It goes well with the theme of the novel: masked heroes may defend people from criminals, but they are as dangerous as the criminals themselves and must be watched closely or they will think they are justified to do anything they want for the greater good.One thing that really threw me... when Dan and Laurie were fighting the robbers in the alley... were they brutally murdering them all? At one point does Laurie stick a knife in the guy's neck? Wouldn't that make them exactly the same as Rorshach? I thought only Ror, the Comedian, and later Manhatten killed.
I would argue its less that they never killed, more that it wasn't their preference. The gangers were looking to have a good time at the expense of their lives and dignity, and perhaps they felt only brutality would turn them back.