How does Napoleon respond to criticism or resistance? Is there a parallel to his behavior in the history of the Soviet Union? If so, what is it?
Napoleon is pretty much a direct representation of Joseph Stalin, the man who was the dictator of the Soviet Union for decades. So there is a parallel to Napoleon's behavior -- Stalin's behavior.
Napoleon responds to criticism or resistance in a couple of main ways. Mostly, he gets rid of those who criticize or resist him. We see this in the case of Snowball -- where Snowball is chased off the farm for resisting Napoleon (Trotsky chased out of the USSR by Stalin). We also see this in how all the animals are executed after the hens rebel against Napoleon.
Another thing that Napoleon does is to have Squealer disseminate propaganda that tries to convince the animals that all is well even when it is not.
The relationship between Stalin and Napoleon is fairly powerful. Consistently embodying the "principles" of Animalism, Napoleon seems to have inherited the mantle from Old Major's call of change. In this light, Old Major is representative of Vladimir Lenin. There are other scenarios where Napoleon is Lenin, himself, and Old Major would be Karl Marx. However, the setting with Napoleon as Stalin is more meaningful because of the lengths that Napoloen goes to in the consolidation of his power, the gap between what is preached by the leaders of the farm and what is practiced, and the cold manner in which perceived opposition is "eliminated" makes the link between Stalin and Napoleon quite relevant.