In Chapter Six, the animals awake to find that the windmill has been destroyed. Initially, Napoleon's reaction is one of shock: he paces "to and fro" in silence, for example, and his tail is "rigid" and "twitchy."
Suddenly, Napoleon declares that it was Snowball who destroyed the windmill. To emphasize his anger at Snowball's betrayal, he roars Snowball's name in a "voice of thunder." He then goes on to pass the sentence of death upon Snowball, "the traitor," and offers half a bushel of apples to any animal who catches him alive.
In reality, the reader knows that it was not Snowball who destroyed the windmill; it was the weather. But, for Napoleon, this is an opportunity to further blacken Snowball's reputation and to convince the animals that Snowball is the enemy. Moreover, by offering a reward for his capture, Napoleon guarantees the loyalty of the animals on the farm since the reward is too good to refuse.