I must also add my daughter has a service dog and 2 cats. She particularly loves her dog, but favors the cat. It is NOT a characteristic of autism to be aggressive towards animals. Just because you have autism does not mean that you are prone to violence.
This a very serious behavior. Is he receiving therapy of any kind? OT, PT, Speech, etc? I really don't think this should be ignored as this aggressive behavior can escalate. My daughter has autism and hates it when I talk to her about her outburts, but I always do. It's very important that she listens to authority figures, just as it is for all children.
As an Asperger person and a big animal lover (like most Aspies), I am horrified that a kid would try to kill a pet. I think there may be much more than his autism to explain this behavior, I would bring him to a psychologist to discuss this episode. In the meantime, make sure he stays away from pets at all times!