I agree that allowing young llamas to live in a herd is a good idea. I think that in order to decide how to prevent aggressive behavior it is important to gain awareness of what causes it. Perhaps more importantly to recognize the early warning signals and act then. It is my experience that aggressive animals happen because a variety of factors coalesce. Llamas are born with different personalities and tendencies.
Humans have different behavioral styles, levels of confidence and experience.
Environmental factors such as multiple owners, age of weaning, herd composition also play a part in the behavior of an llama. A young male raised alone with no llama babies to play with. The owners have young children. The children encourage aggressive play behavior directed at humans. You get the picture.