What Hitler did and what Mao did was terrible. But it was also inevitable. If we suppressed every horrible historical event, the human race would learn nothing. Every event, every death, can be a learning experience.
If you went back in time to keep a child from burning his hand, he would just keep touching the stove till he burned his hand. There's no fighting the inevitable. It's like fighting media piracy in 2012. It's like playing whack a mole.
There are some truths to these statements, but while some of the most dreadful mistakes made in history are not worth changing and could create grave consequences if changed, it is also important to note that time and time again, history repeats itself. There are many mistakes that people never learn from, and those mistakes are constantly repeated.
Now, I'm not saying I plan on going back and time to kill Hitler. In fact, I feel one should analyze more practical methods of how one should change history for the better, instead of making a hasty move like going back in time just to kill Hitler.
For instance, my plan on dealing with Hitler and the Holocaust is this: Let Hitler exterminate 6 million European Jews, but there's a catch. In the aftermath of World War II, surviving European Jews will become missionaries for their faith, promoting Judaism to gentiles. Upon converting to Judaism, converts will not be forced to reject the divinity of Jesus or obey the old Jewish dietary laws and restrictions, and thus the converts will be able to exercise freedom of thought in certain aspects while still adhering to core Jewish religious traditions.
While there is no question that many European gentiles will reject converting to Judaism, many others will do the opposite. And this will eventually reimburse the lost Jewish population in Europe. And in addition to all of this, 6 million Jews will not be as major of a loss to Europe as it is in current history, as I plan on there being more millions of European Jews in the pre-Holocaust era.
As for the 6 million European gentiles Hitler exterminated, that will be reimbursed as well, as a result of reproduction, and that includes having bigger families among the peoples of Europe.
So no, I don't plan on killing Hitler before his suicide, and I would encourage others to leave it that way. Preventing the premature deaths of Elvis and John Lennon is another story...