Shane is a self centered ass - always has been. He has always believed himself a hero, but he's not. He can use the concept of survival as his excuse, but self sacrifice is what a hero does, not sacrifice others for their own purposes. Yes, he loves Carl, but others love Otis. It's not his place to decide, and in reality he was saving his own ass, not Carl's. It's convienent for him that one might lead to the other.
Can he manage to live with the cognitive desonance that this sets up? He thinks of himself a hero (even when he tries to rape Laurie and considers shooting Rick, he finds a way to rationalize the behavior), but can he manage to rationalze it this time? Deep down, it wasn't Carl's survival that mattered to Shane (and he wouldn't even be able to admit this to himself) but Laurie's approval. And he got it when she told him to stay.
Whether anyone would do what he did or not is always debatable, but like devil said, what matters is how he handles it. A truly normal person who still had their humanity would agonize over it for the rest of their lives. They'd never be 'good' with it - you can't be and remain human. You'd always carry guilt. But if you get yourself to a place where that behaviour wasn't just necessary in that instant, but acceptable and 'right', and can feel you have a completely clear conscience, you become a real threat to everyone around you.