I don't think Shane was being completely altruistic. As much as he probably wanted to save Carl, I think it had more to do with his own survival than anything else.
We've seen how self-involved Shane can be, from still lusting after Lori even after Rick's return to nearly shooting Rick (almost literally in the back). I would have thought maybe he had moved on from Lori in this new season, but given the conversation he had with Lori in the first episode, he's clearly still wallowing in self-pity.
I think the show is building Shane as the kind of character that values himself above others, but that doesn't mean he's an outright horrible character (and even that sense of self-value is very realistic and even appropriate given the circumstances). He obviously cares about the other members of the group, including Rick, Lori and Carl, and it was very selfless of him to venture out with Otis (even if I suspect some of that might have been to appeal to Lori; or, as others have said, to be seen as a hero in her eyes).
If anything, Shane is becoming a very multi-faceted and complex character. He's not necessarily evil, but I think ultimately he's going to possibly start causing problems for the group in some shape or form some point down the line. Also, I thought shaving his head like that would attract more attention than just having that bald spot. He could easily explain that (zombies), but he's going to have a slightly more difficult time explaining his shaved head (unless he claims he did it for vanity reasons or whatever). I'm sure shaving his head was some kind of visual indicator for the direction Shane is heading, but it didn't make much sense in my eyes for practical reasons.