When one reviews Snowball's activities and what he said, it becomes evident that he had all the animals' interests at heart. He selflessly and tirelessly dedicated himself to their cause, which was to ensure a better life for all animals, free from bondage, abuse and exploitation. This aspect is clearly illustrated when he gets involved in the education of the animals, especially the less intelligent ones, his indefatigable work with his committees to ensure that the farm is run better and that everyone is involved, and his plans to build a windmill which would be used to generate electricity and make life much more comfortable and easier for everyone on the farm.
There is no evidence that Snowball bore any malice. He is depicted as good and is the perfect foil for Napoleon, who only had his own interests at heart. Although he occupied a position of authority, Snowball did not abuse his power to manipulate and exploit the other animals, whereas Napoleon did. In spite of benefiting from the windfall apples and the milk which disappeared, just as the other pigs did, Snowball never sought greater privilege or comfort for himself. He clearly wanted all the animals to, on the whole, benefit equally.
However, Snowball's kindness and hard work all came to nothing for he stood in the way of Napoleon's ambition. Napoleon wanted sole power and would ruthlessly pursue his goal. He was sly and carefully planned his ascension to become a dictator. He chose an opportune moment to ruthlessly get rid of Snowball by setting his dogs on him and chase him off the farm. He then demonized him and eventually turned the animals against him so that he was despised and seen as a traitor who was intent on destroying everything the animals hoped to achieve.
Napoleon used propaganda, manipulation, ruthless violence and threats to assert his authority and soon the animals found themselves even worse off than they had been under Jones and his men.
I believe that the lesson to be learnt from Snowball is that good can be overwhelmed by evil if those who do good are not wary of the malevolence lurking within their very midst. Snowball believed in himself and never guarded or acted against Napoleon. He was quite naive and never questioned Napoleon's motives. Although they disagreed most vehemently during meetings, Snowball never challenged Napoleon about his motives like, for example, about his purpose with the nine puppies that he had removed from their mothers once they were weaned.
The dogs became Napoleon's trump card and he used them to serve his insidious purpose. One can be sure that if Snowball had insisted that they be raised by their mothers and socialize with the general populace, the outcome would have been quite different. Bluebell and Jessie would definitely have supported him in this regard. His innate goodness did not allow him to suspect any malice on Napoleon's part and that was his greatest flaw.