Tyranny exists when a single ruler or a small group of rulers exert unlimited power over citizens in ways that are unfair, cruel, and oppressive. Famous historical leaders commonly labeled as tyrants include Oliver Cromwell, Fu Sheng, Ivan IV, and Leopold II. Check out the link I'm including below to examine their commonalities.

Napoleon is often classified as a tyrant in Animal Farm. When he and Snowball meet to debate the construction of a windmill, Snowball outlines his points in earnest, hoping to engage in a meaningful discussion. Napoleon has no intention of debating Snowball and simply states that the idea is "nonsense." The animals notice that he seems "indifferent" about his argument or their reaction to it, and the reason soon becomes clear as nine enormous dogs burst into the barn to attack Snowball. After running him off, the dogs faithfully return to the side of their leader, Napoleon.

It becomes clear that Napoleon intends to use coercion to force his will on the other animals, which is an indication of tyranny. He is unwilling to accept blame for his shortcomings and influences the way the animals perceive him by making Snowball seem incompetent and even dangerous. When the windmill is destroyed during a storm, Napoleon uses the opportunity to place the blame squarely on Snowball:

Comrades ... Do you know the enemy who has come in the night and overthrown our windmill? SNOWBALL! ... Snowball has done this thing! In sheer malignity, thinking to set back our plans and avenge himself for his ignominious expulsion, this traitor has crept here under cover of night and destroyed our work of nearly a year. Comrades, here and now I pronounce the death sentence upon Snowball.

Though the idea is preposterous, Napoleon convinces the animals to agree with him. In order to serve his own interests, Napoleon proves himself tyrannical by destroying truth and peace, willing to use intimidation and force to bolster his own power.