It's a tough call in this one.
James, the situation is that Otis (the one who got sacrificed) accidentally had shot the kid (Carl), and so when they needed medical supplies for the kid, he felt bound to set things right and go get the supplies. Shane went with him. Throughout the excursion to get the supplies, there were several moments where it looked like they might leave one of them behind, Shane even yelling at Otis to get going, but Otis wouldn't leave him. Then, when they were down to their last round of ammo each, Shane shot Otis in the leg to leave him as a distraction for the zombies, so he could take the supplies back to save the kid. If he hadn't they probably would have both died, and so would the kid.
It's a tough call and I have a lot of mixed feelings about it. Shane is a "get things done" kind of guy, and sometimes a loose cannon, but mostly has his heart in the right place. I think he's sort of mentally adopted the kid too.
The biggest problem I have is that he didn't give Otis the choice. Otis was such a good guy, and wanted to set things right, that he might have chosen to stay back, but Shane made a quick and ruthless tactical choice and decided for him.
The thing that's not clear, is how much of Shane's decision was motivated by saving the kid, and how much was self-preservation. There's nothing wrong with self-preservation of course, but how would we feel about Shane's choice had there not been a kid's life at stake? Or if Otis wasn't the one responsible (accidental as the situation was)?
In the end, he saved the kid, and he's going to be haunted by what he did. Yet another secret he has to keep from his friends. I'm wondering if one or two of the folks in that farm house might join the group, perhaps the attractive young lady? He'll have to keep it secret from them for sure.
What he did was both wrong and right. He did something unthinkable for the greater good. But also selfishly. Wow.
This is why I love this show. The writing is awesome.