To answer this question, you have to understand the global political climate at the time the Constitution was created and ratified: Tyrants were the norm.
The Patriots had just fought a bloody war to free themselves from what they saw as the arbitrary Tyranny of the British government.
The British parliament at that time was subject to a King's veto (not just symbolic as it is now), and the House of Commons was subservient to the upper House of Lords (essentially an unelected Senate comprised of very wealthy noblemen).
From the Common Sense pamphlet written by Tomas Paine in 1776 (one of the most influential documents in promoting the Revolution):
How came the king by a power which the people are afraid to trust, and always obliged to check? Such a power could not be the gift of a wise people, neither can any power, which needs checking, be from God; yet the provision,which the constitution makes, supposes such a power to exist.
All of the countries in Europe were ruled by some form of Monarchy, which the Founding Fathers saw as tyranny.
With this context in mind, now look back to the Constitution. The main function of the Constitution was to create a government which would be the least susceptible to corruption and tyranny.
If any men in history were ever skeptical of the corruption of a government, it was the Founding Fathers. Of course they anticipated that there would be unjust and corrupt leaders, they would have fully expected it.
Of course they realized that men in the future would try to twist the meaning of the Constitution. They just hoped that future generations, and the system of checks and balances, would be strong enough to face those challenges