Violates the principle of benevolence.
'So is the superior man affected towards animals, that, having seen them alive, he cannot bear to see them die; having heard their dying cries, he cannot bear to eat their flesh.' (Mencius 1.7) To abuse animals is to violate the principle of benevolence, which is based on the feeling of commiseration.
Moreover, humans should assist in the nourishing powers of the earth - and that includes protecting and respecting our animals. 'Able to give its full development to the nature of other men, he can give their full development to the natures of animals and things. Able to give their full development to the natures of creatures and things, he can assist the transforming and nourishing powers of Heaven and Earth. Able to assist the transforming and nourishing powers of Heaven and Earth, he may with Heaven and Earth form a ternion.' (Doctrine of the Mean 23)
'In regard to inferior creatures, the superior man is kind to them, but not loving.' (Mencius 13.45)