1. Otis and Shane were
complete strangers. Complete strangers in a post-apocalyptic world
2. Otis was the reason they were in that predicament because he shot Carl.
3. Otis and Shane were exhausted/tired from carrying those supplies. They were going too slow. The walkers were catching up.
4. Those zombies would have followed them all the way back to camp.
5. If Shane sacrificed himself, Otis being overweight and out of shape wasn't a safe bet making it back.
6. Don't think for a second Otis wouldn't have left Shane if it came down to it
7. Rule #1. Cardio.
It was grimy and evil. But Shane made the call he felt he had to make.