You would of thought Shane would of shot Otis in the head to let him off the pain of being eaten alive.
Some of the zombies would probably be more interested in the limping guy running away than the completely still body on the ground, and continue to chase Shane. They can smell fresh food but they have to be pretty damn close. They seem to prioritise the living, and have been known to stop eating a dead victim if they catch a glimpse of another potential target.
There's no way they were going to ignore a screaming, begging fighting, immobile target. He was always going to be louder and more enticing than Shane, who was able to pretty much slip away.