Why does anyone want power? There is an innate proclivity in many to desire control over others and to have an exalted position in their societies. While ostensibly concerned for the other animals, Napoleon manipulates circumstances and the rules established after the revolution. In this way, he does what he wants and has anything that he desires.
Once Napoleon banishes Snowball, he has absolute power, and as the old adage goes, "Absolute Power corrupts absolutely." He trains dogs as his enforcers and goes to live in Mr. Jones's house. The commandments are revised; his propagandist Squealer reinterprets the law so that the other animals will not object.
Orwell's pointed criticism of the Stalin regime is evident in this allegory. The conflict between Snowball and Napoleon represents the Lenin/Trotsky conflict in Russia with Lenin prevailing. Stalin then took over after Lenin; he was a brutal dictator.