The Buddha clearly told us that meat eaters would become carnivores such as lion, tiger, and leopard if they were to descend to the animal realm. This can be inferred through logic as well. In the animal realm, there are only two categories of food: meat or non-meat (vegetables, fruits and nuts). At that point, because of the deeply ingrained tendency to eat meat (habitual tendencies can wield great power), meat eaters will become carnivores who can only kill to survive. It is stated in the Abhidharma-kosha-shastra that there are three types of killing—killing born of greed, ignorance and anger. To kill for food is one born of greed.
[Excerpt from Luminous Wisdom Book Series 1: The Right View ~Why Vegetarian?]