Essentially, Swift thought all of human nature as deplorable. Swift was a believer in the Protestant idea that man is by nature sinful, having fallen from the Garden of Eden. He believed that while man is rational, this rationality does not necessarily lead to good. Therefore, one should not consider rationality the greatest human quality. Swift felt it was human nature to sin: to be deceitful, cruel, selfish, materialistic, vain, foolish, and otherwise flawed. For him, rationality and institutions such as governments, churches, and social structures exist solely to control man's tendency to sin, to keep him in line.
These beliefs are evident throughout Gulliver's Travels. For example, Gulliver meets his physical and moral inferiors in the Lilliputians, and sees that they have well-thought-out but illogical and even unethical ideas about justice, schooling children, and choosing political leaders. On the contrary, his physical and moral superiors, the Brobdingnagians, do not suffer war or strife because their political and social structures are far superior to England's. In the novel, Gulliver encounters societies that ridicule the human condition, either by mocking it or imitating it. Yet there is no suggestion of how we can make it right again, or how we can overcome our nature. Swift suggests that we can never return to that state of perfection in the idyllic garden because it is the human condition to sin, but we can at least rise above our Yahoo-ness.
One aspect that Swift particularly focused on in his satire was politics. Swift chose to ridicule the worst aspects of politics in Gulliver's Travels. Most of this is found in Part I, which mirrors the events in England in Swift's day. The petty Lilliputian emperor represents the worst kind of governor, pompous and too easily influenced by his counselors' selfish ambitions. Swift also explores the duties and purpose of government in Parts I, II, and IV. By having Gulliver discuss his system of government and compare it to the ones he discovers, Swift raises questions about government's role in public education, provisions for the poor, and distribution of wealth. Part of what makes Gulliver's Travels so provocative and timely even today is that Swift doesn't provide simplistic answers to these questions. He shows that human natuer is complicated, and that we all demonstrate the best and worst of humanity at times.