How and why did Napoleon kill the animals in Animal Farm?

Remember that 'Animal Farm' is not just a fable about animals but an allegory to the Russian revolution. Napoleon represents Stalin, so his execution of animals would correspond to the political purges of the Bolsheviks once they gained control.

During this period, he orders the execution of several of the animals after coercing their "confessions" of wrongdoing. He then commands the building of a second, stronger windmill, while severely cutting rations of the animals, except those of the pigs and dogs.

answer; (external conflict)