Of all the creatures that were made he [man] is the most detestable. Of the entire brood he is the only one — the solitary one — that possesses malice. That is the basest of all instincts, passions, vices — the most hateful... He is the only creature that inflicts pain for sport, knowing it to be pain... Also — in all the list he is the only creature that has a nasty mind.
Vol. II, p. 7. - Mark Twain's Autobiography (1924)