There are many examples of such leaders who promised something and after coming into power they did something else, something that people never imagined before. Let's consider the case of Russian leader Stalin, who was believed to be the God for Russians. But as soon as he came into power, around 10-15 million Russians were exterminated during 1920-30. Germany was in the strong financial condition before Adolf Hitler came into power. He promised good infrastructure, good living standards of the society from which he belonged, their secured future. Other religion's people also forced to believe him so much that no one opposed. But you are aware of what Hitler did with Jews. Many millions Jews were exterminated. Another example of such crucial leader is Mao from China. Same promises he made to the people of China. But during his reign, around 60-65 million Chinese were exterminated. This has been recorded as the biggest manslaughter, genocide in the history. There are many more leaders in this category. My mind is not that much sharp to recall their names :)
There are many good leaders emerged who really did something very good for the society, and they were true to their promises made.
From above examples, I have found two similarities in the above leaders. One is, they were very crucial to the other societies (and not from they belonged) and other is, they were great supporters of Eugenics! (This is responsible for most of the genocides in 20th century).

I am worried about the US president Barack Obama, as he is also a great leader, belongs to Blacks in USA (blacks being suffered a lot during 20th Century in USA). So many people still cannot believe him. They still think the history can repeat any time. Worst could be the extermination of Whites for the welfare of Blacks! (I pray god not to repeat the history!) I am
not negative thinker, but trying to put the -ve side as well. How Obama deals with Eugenics, matters a lot. I am also worried about our Indian Leaders who will be in power after this year's polls. India is not yet in the list of those countries that had practiced such crucial ways of Eugenics. If I had been one of the great leaders, then I would have died protesting against such genocide techniques. If a person is not mentally fit, then it’s not their mistake. It’s our duty to let them live safer life in the society, rather then letting them go and die.
Let's pray god for long life of India and its great citizens, and keep India Eugenics-free !