One of the troubling aspects about Animal Farm is that we see that, in the exercise of political power, good leadership is less important than the willingness to use force, coercion, and duplicity. By the end of the book, Napoleon has become so powerful that it is difficult to distinguish him from the humans. He does so by eliminating his rivals, particularly the brave and idealistic Snowball, squelching all dissent using the ferocious dogs he trains as killers from their puppyhood, and by manipulating information through the skilled propagandist Squealer. After a while, he leads by cult of personality, as the following passage describes:
Napoleon was now never spoken of simply as “Napoleon.” He was always referred to in formal style as “our Leader, Comrade Napoleon,” and this pigs liked to invent for him such titles as Father of All Animals, Terror of Mankind, Protector of the Sheep-fold, Ducklings’ Friend, and the like.
So while we might not view Napoleon as a good leader as such (perhaps Snowball would be the closest to deserving of this title) Orwell's point is that he rises to power because of a willingness of the animals to follow leaders. He is brutal, dishonest, and ambitious, and unfortunately, these people (or pigs) are the types that often become powerful. Orwell is not primarily concerned with providing case studies in good leadership but is rather about the perils of political power in general.