There's a reason you don't want your prom queen to also be your school president: absolute power corrupts absolutely, and pretty soon she'll be sending out her minions to stake out the best parking spot. In Animal Farm, the pigs no sooner weasel their way into power than they start taking milk for themselves—and pretty soon, they've moved on to harder stuff. Like whiskey. So, is there any hope? Does Orwell offer any model of government that doesn't just get corrupted?
Although they definitely get worse as the story progresses, the pigs are greedy from the start.
When Napoleon takes over, Animal Farm is doomed. Snowball was no angel, but he was a sound leader. Napoleon is just bad to the (delicious) bone.