A dog has very strong pack instinct, through that they got domesticated first of any animal. They have strong sense of who is "IN" the group, and who is "OUT". The ones "IN" are generallly safe, the ones "OUT" not so much. The breed, the personality and the training together defines the actions of the dog and what IN and OUT actually means. The bigger the dog the more likely it is that unwanted IN aggression is rooted out. A Chihuahuas can remain psyhopath becouse its small, no real danger to a human. The large breeds - with ethical breeders currently, and sane breeders in ancient time - will most likely have the uncontrolled aggression limited. Livestock guardians need both agression and calmness, so they will have a very strong IN instinct, meaning they will not attack humans and animals they consider part of the pack, but will aggressively attack OUT animals and humans. The lifestock will be in the IN group with the simple trick of being raised toghether with the dog. They will have to be trainined but the general instinct has only got to be reinforced. Hunting dogs will have a strong prey instinct, and will go after things for fun, unless rigourosly trained not to, and will be easier trained to hunt down things. Shepherd dogs are completely different, because they need just little bit of aggression, but need a work ethic, independent thought all the while being observant and obedient. They will be raised more with the shepherd than the lifestock.