If you are an eighteen or nineteen-year-old with little education, as is often the case, and you're put in charge of many, many people on the other end of the world, you have absolute power and you're not prepared for it.
It is not true that the legislator has absolute power over our persons and property, since they pre-exist, and his work is only to secure them from injury. It is not true that the mission of the law is to regulate our consciences, our ideas, our will, our education, our sentiments, our works, our exchanges, our gifts, our enjoyments. Its mission is to prevent the rights of one from interfering with those of another, in any one of these things.
The mistakes (of leaders) are amplified by the numbers who follow them without question. Charismatic leaders tend to build up followings, power structures and these power structures tend to be taken over by people who are corruptible. I don't think that the old saw about 'power corrupts and absolute power corrupts absolutely' is accurate: I think power attracts the corruptible.
The fate of every democracy, of every government based on the sovereignty of the people, depends on the choices it makes between these opposite principles, absolute power on the one hand, and on the other the restraints of legality and the authority of tradition.