Suggest a reason why Orwell needed to write about Snowball

Snowball represents a responsible intelligent leader that could have come close to realizing Old Major's dream of an animal utopia. Instead a more ruthless controlling dictator not only unseated him but banished him. This is a familiar story as far as power struggles go. Orwell, however, meant Animal farm to be an allegory of Stalinist Russia. Snowball, who represents Leon Trotsky, is a progressive politician and aims to improve Animal Farm with a windmill and other technological advances, but Napoleon expels him before he can do so. In his absence, Snowball comes to represent an abstract idea of evil. The animals blame misfortunes on him, including the windmill’s destruction, and entertain the idea that he is lurking on one of the neighboring farms, plotting revenge. Napoleon uses the animals’ fear of Snowball to create new propaganda and changes history to make it seem as though Snowball was always a spy and a traitor.