It was survival and nothing else. The fact is they were moving too slow and the walkers were gaining on them. there was absolutely NO WAY they were going to make it back together. one of them had to stay and hold the walkers off so the other could escape, and shane even OFFERED to do it, but otis wanted to try and push on together. shane saw it just wasn't happening, so he did what he did. If he kept him alive and they kept running, they would have been caught and they both would have been killed, and carl would have died because there would be no medical supplies.
people hating him and calling him a murderer for that don't seem to understand that the rules of the old world do not apply in this one. there is no good and evil, there is no right and wrong. What might have once been considered murder is now considered survival, and nothing more. you do whatever you have to to stay alive.
Or you know, Shane could've just ran faster. Even if Otis wouldn't have been able to keep up, he'd still have a fighting chance.