Snowball's idea for the windmill is a classic example of sacrifice for the greater good; he truly believes that if all the animals get together and work hard, giving up temporary luxury while the windmill is built, they will be able to reap the benefits for years to come. However, Napoleon knows that if the animals actually get the leisure time they were promised, they will start wondering why the pigs aren't doing as much work as the others, while enjoying such great benefits as extra milk and apples. Just as Snowball finishes speaking:
...Napoleon stood up and, casting a peculiar sidelong look at Snowball, uttered a high-pitched whimper...
At this there was a terrible baying sound outside, and nine enormous dogs wearing brass-studded collars came bounding into the barn. They dashed straight for Snowball, who only sprang from his place just in time to escape their snapping jaws.
(Orwell, Animal Farm, msxnet.org)
Like the expelling of Leon Trotsky by Stalin, Snowball is driven from the farm so that Napoleon can continue gaining personal power. He would have preferred that Snowball be killed, but afterwards, Napoleon realizes that he can use Snowball as a bogyman for the other animals, an imaginary enemy that he, the brave leader, can pretend to be fighting. This allows him to push the animals even harder, while doing nothing himself, in the pretense of "working hard" to keep Snowball from coming back and taking over.