Meh, just lost everything I wrote. In short, the fact Shane shot Otis in the leg and not the head to make it a clean kill and prevent possible zombification if he somehow didn't get completely eaten says it all. I also don't buy that it's unbelievable that this is just what he's like. We have seen hints of it the whole way through the show, and how often do people live and love alongside sociopaths and serial killers and innumerable other mental disorders without realizing it until something in them cracks?
I personally think they've done a good job of evoking such visceral emotion from the audience, even if it's loathing and dislike, toward a character.
Also, for those of tou who enjoy Max Brooks, you may get a kick out of So Now You're a Zombie: A Handbook for the Newly Undead by John Austin.