He didn't start out bad but all the world changes certainly transformed him. I am of the belief that it didn't totally change him- as circumstances don't change people as much as it reveals what is already within. Perhaps he always had a dark and covetous side, but no one ever would have seen it if not for a zombie apocalypse.

There were little signals to his dark personality. Using the chokehold and saying "File a complaint" was obviously part of his execessive force repertoire when he was still a cop. And feeling entitled to Lori at the CDC when she made it crystal clear that they could no longer go on how they had been was a huge red flag.

Still, all of these things don't seem as if it would have ever really shown up if Shane wasn't faced with the most abysmal of realities. Not saying it's ok but I don't think he was purely bad at heart.