"Compassion with animals is intimately associated with the goodness of character" it's a great line from Schopenhauer and he says that the opposite is also true. That a bad person is somebody who is cruel towards animals, and the basic idea is that the manner in which you treat animals reflects upon yourself if you show kindness and patience and tenderness towards animals. It means that you're probably kind and graceful and tender towards yourself. Whereas if you take it out on an animal, if you're angry with an animal, if you accuse them of things that have more to do with your own human emotions than with their animal state of nature. It says more about you than about them.

So a sign of a good person is someone who treats animals well.