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.
Shane did make an attempt to get Otis to take the bags, presumably on the assumption that he thought Otis had the better chance to get away. So the way I read the situation was about as tragic as can be imagined: there was no way both of them were going to survive and the one to survive would have to be the "worse" guy.
Carl's life depended on one of them staying behind as bait. Shane offered to be bait, Otis refused, because he's a better man than Shane, perhaps. That batted the ball into Shane's court, and he was the one who made the only decision he could have: since Otis wouldn't leave Shane as bait, Shane was forced to turn Otis into bait.
But geeze, couldn't Shane have shot the guy cleanly in the head? I guess we can chalk that up to Shane not being able to bring himself to kill an innocent person outright, being a sherrif and all, but it would have been the more merciful, not to mention efficient, option.
Otis almost kept Shane from getting away. I really can't see Shane being cold-blooded enough in that situation to rationally assess whether Otis would be better bait alive or dead. Shooting Otis in the foot is as much as he could bring himself to do, in a situation where there was no time to think anything through.
I don't think Shane is a bad guy at all, at least not right now. But I think he's going to become a case study in the burden that excessive guilt can place on the psyche. He'll never shake Lori's suspicion that he lied about Rick's death because it's impossible for Shane himself to ever know whether he was deluding himself that Rick had no heartbeat in the hospital. To try to overcome that guilt, he's heaped something worse on himself. I think we're going into Macbeth territory with him. Specifically, this:
"I am in blood
Stepp'd in so far, that, should I wade no more,
Returning were as tedious as go o'er."
, Scene IV
Also, I thought shaving his head like that would attract more attention than just having that bald spot.
Something like that can be chalked up to the emotional trauma of seeing Otis eaten alive, even if nobody knows Shane was to blame, not to mention all the other trauma he and everyone else has endured. All these people are on edge emotionally and that can manifest itself in odd ways.
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.
I see it as mainly psychological rather than practical. He could tell everyone a zombie pulled out his hair in trying to grab him. Shaving his head is a recognition of guilt and penitence or a confirmation that he's planning to just keep wading across that river, Macbeth-style, or most interestingly: it's both.