Rick was in a hospital in a coma when the zombie virus emerged and largely played out, it seems, and is just regaining consciousness when the series begins.

In season 1 and season 2, Shayne's far more experienced with surviving in the zombie world. We'll see where Rick's morals are after he has comparable experience with surviving in the post-apocalypse.

Shayne's head-shearing moment of truth/despair:

But for Shayne (and Doctor Hershel, of course), Carl probably would not have survived the gunshot surgery. In the episode where Shayne and Otis (the hunter who accidentally shot Carl) go to the zombie-infested facility to get medical supplies for Carl's surgery, Shayne was certainly being self-interested in disabling Otis to occupy the pursuing zombies. But if I were Rick, Shayne's the one I'd have entrusted to get the supplies back in time for the surgery, too.

Truth be told, given a choice between being the evening hors d'oeuvres for a pack of famished zombies, or making some stranger (who'd just shot my godson) the meal.... well, my altruism would be sorely tested.

I think Shayne's being too hard on himself for the decision he made to sacrifice Otis (the guy who, after all, shot little Carl).

All in all, if I were trying to survive their zombie-infested world, I'd be delighted to join a group that had Shayne, Rick and Daryl in it. And Andrea, too, now that she's a sharpshooter.