There was a wonderful program on this on NatGeo channel this morning about how man made dogs out of wolves over the millenia. How certain wolves learned that if they attached themselves to a human camp they'd feast on the scraps and it was easier than hunting. In turn, the humans noticed that these wolf/dogs would alert to predators approaching the camp. And so began the symbiotic relationship between man and dog. Very good program and worth watching (2 hrs.) when they repeat it.
That makes slaughter much easier.
They were undoubtedly domesticated for the purpose of food. As the smarter ones managed to survive an untimely end, they began working for a living.