Something just happens to people when they get to much power. They start to think they are better than everyone else. Some of it comes from the lower level people themselves. They kiss the people in powers feet to much. Which allows the powerful to think that they should be treated like Gods. When they start to get treated like a God, they believe they can do anything they want, and get away with it.
Absolute power is definitely corrupting in some rulers. For example, if a person is granted absolute ruling over another country, he or she becomes completely infatuated with himself or herself. If someone has absolute power, they can do anything. With no opposition, a person can destroy the world if he or she pleases. If the president had absolute power right now, there is no doubt he would only do things the way he wants them to be. We need checks and balances.
Anyone with complete power is going to end up corrupted. People need checks and balances in order to not get corrupted. There needs to be people around those in power that are not simply yes-men, who actually challenge the actions of those in power. Absolute power corrupts even the best people.
I believe in most cases absolute power does corrupt. We really don't have any proof of it not corrupting a person or any entity. Any case that involves absolute power tends to show corruption at some point or another. This is why absolute power is looked down upon by most people.
No human can be trusted. That is why we need a dictatorship so we have to trust fewer people. I would rather have 1 or 2 evil people in charge than hundreds of evil people in charge. Weak democracies will usually become dictatorships and that is for the better of the citizens.