I've rewatched the scene and Lori's reaction is not due to Rick killing Shane but rather when she learned that was Carl who put Shane down. Rewatch the scene and you'll see that Sarah Wayne Callies has like ten different facial expressions and "disgust" appears once Rick tells her it was Carl who pulled the trigger.
Yeah I think that was the biggest part of it. Just the fact that her son had to be a part of such a horrible act in the first place.
In the comic, the introduction of a walker toting woman with a katana who kicks ass and takes names seemed to fit right in.. She was almost supernatural in her ability to fight and "tame" her walkers...
My concern is how this will translate onto the screen where life seems a bit more realistic... Or maybe I just need to go read the comic again to see how this played out.. It's been more than a year since I read it.
I haven't read the comic, but I don't think it would be that hard to make her fit. They'll probably just downplay some of her fighting abilities a bit, and have her simply be another badass Daryl-type-- only with a sword instead of a crossbow.