Come on now. Lori wasn't being dumb. She was being devious. She knew Rick could take him and she was turning up the heat so the kettle would boil. She wanted Shane out of the way so he wouldn't endanger Carl with his erratic behavior. That in itself is pretty dumb, but I think it came from more of a plotting place than a just being too dumb to realize Shane would lose it place.
There's a guy coming up at some point called The Governor that I won't say anything spoiler-y about, but will just say that he very strongly illustrates why a society built on the mindset that Shane had in the show does not work for rebuilding anything.