The directing was bad in that scene. Basically it should have been made obvious that they both couldnt get back alive. However the way it was filmed it looked like if shane hadnt killed otis they could have both made it back to the car with time to spare. The car wasnt far away and the zombies were too far back to be a problem. The fight scene where he was taking the backpack off took ages. Realistically the zombies should have made it too them both before he could get the back pack.