Do predatory animals gain bad karma? To my understanding, the killing of any animal will produce bad karma to whoever or whatever it committed the murder. For humans, I can understand that we have the power to go vegan and let animals be by themselves, it is our nature. But for predatory animals, their instinct is that they must kill for food. If they don't, they simply can't go vegan, they are going to die.

So do predatory animals get bad karma for killing, a natural instinct for survival?
Yes. Since the question is reasoning from the point of a belief in the realms, I'll answer it as one.

In the realm of animals, it is difficult to earn good karma since it is assumed that animals life brutish lives of killing and savagery.  In Tibetan Buddhism, contemplating how lucky we are to be human where it is possible to practice Buddhism is a fundamental practice. In a typical contemplation, the unfortunate lot of the animals is a common theme.

As a side note from someone who sees the realms as myth and not necessarily the most valuable ones, I would speculate that if we were, say rather smart cats, and the Buddha were a cat, then we still could choose to eat fewer animals, be more humane in their raising and slaughter, or slowly switch from animals to plants. My own cat eats an omnivore diet, a product of how pet food is sold in the US. He eat enough corn gluten that when I'm eating corn, he wants to eat it. Cats are poorly adapted to a vegan diet, but do fine on an omnivore diet.

In biology, omnivores, carnivores and herbivores often evolve into a different category, although they ordinarily only make one jump at a time, from carnivore to omnivore, then to herbivore.