Heterodontosaurus is the signature heterodontosaur, a large group of genera which includes many small ornithopods, all characterized by bipedal posture and long canine-like teeth in the top and bottom of the jaws (which only the bottom ones would have been plainly obvious in life. They may have these teeth for similar reasons as musk deer and other modern hooved mammals--defense, display, and sometimes to help forage. Animals with big, scary fangs are not necessarily carnivores! Especially not these ones. They wouldn't even be particularly dangerous if threatened since Heterodontosaurus is scarcely larger than a small dog.