I liked Shane at the beginning. He embraced what needed to be done quicker and easier than Rick did. The problem is, once he turned that switch, he was hell bent and going downhill fast.
The situation with Rick's wife, and who's kid is it afterall, was going to make that little menage' a sh*t show in nothing flat and the group would have suffered.
And well, he did snap that kid's neck in the woods for no real good reason.