Snowball, was in fact, innocent. He was a victim of Napoleon's propaganda and manipulation. He was actually more committed to the revolutionary ideal than any other animal on the farm. It was he who had the best interests of all the animals at heart and tried to, through his various committees, establish a system on the farm which would benefit all and not only himself or the pigs. Although he benefited somewhat more than the other animals for being a pig, such as with the milk, the apples and not performing any physical labour, it was clear that he wanted the farm to be successful for all.
It was for this reason that Snowball was very popular. This did not sit well with Napoleon because he had an ulterior motive. In debates, he would always be challenged by Snowball.
Snowball and Napoleon were by far the most active in the debates. But it was noticed that these two were never in agreement: whatever suggestion either of them made, the other could be counted on to oppose it. Even when it was resolved--a thing no one could object to in itself--to set aside the small paddock behind the orchard as a home of rest for animals who were past work, there was a stormy debate over the correct retiring age for each class of animal.
Napoleon knew that if he should do anything that did not seem to the benefit of all, Snowball would challenge him. It is for this reason that he had to get rid of him. His opportunity came during one fiery debate about the windmill and the defence of the farm. Snowball had meticulously drawn up plans and was at the point of winning the vote to implement his ideas, when Napoleon acted. He let out a whimper and the nine puppies he had taken from Jessie and Bluebell which were now grown, bounded out and attacked Snowball, who had to run for his life. Snowball barely made it through the hedge, but escaped.
Once Snowball was out of the way, Napoleon could do as he wished. In order to demonise Snowball, he started spreading propaganda about him, by using Squealer. All sorts of rumours were spread about Snowball, with Squealer exclaiming gleefully that what Napoleon had been using were 'tactics'. Snowball was a criminal and nothing more. When the completed windmill was blown down by raging winds, Snowball was blamed for destroying it. Napoleon pronounced the death sentence on Snowball and promised to reward any animal who brought him to justice.
The animals were obviously too dim-witted and gullible to challenge Napoleon and some of them even expressed outrage at Snowball's betrayal. Evidence was planted everywhere to further illustrate Snowball's heinous acts. Napoleon always took the lead in identifying evidence supposedly left by Snowball in his supposedly pernicious attempts to jeopardise the farm. It became so bad that Snowball was blamed for practically everything that went wrong on the farm.
It was even claimed that Snowball had been in cahoots with Mr Jones right from the start and that he had sold himself to Frederick of Pinchfield farm. Squealer used clever language and his skill for making speeches to further convince the animals of Snowball's malignant activities.
In the end, Napoleon had destroyed everything good about Snowball's memory. The animals were entirely convinced about his guilt and his evil and so successful was this brainwashing and propaganda that some had even confessed to have been secretly working with him! Upon their confessions, they were summarily executed by the dogs.
The irony is that Napoleon, who was the real threat, had so implicated a guiltless, caring, individual that the animals almost literally ate out of his hands and felt that Napoleon had their best interests at heart, best illustrated by Boxer's naïve proclamation that, 'Napoleon is always right.' Napoleon had succeeded in gaining the animals absolute trust and, with it, absolute power. He was, at this stage, already a dictator.