The Necessity of Genocide
The most explicit parallel between Hitler and Ender is that they're both genocides. Hitler, of course, ordered the death of millions of Jews, Slavs, homosexuals, physically and mentally handicapped persons, and so on. Ender exterminated an entire intelligent species. Most people, I hope, agree that mass murder, much less genocide, is quite indefensible. Yet, as we follow Ender's life after he wipes out the Buggers, we're invited to understand and forgive his actions.
Why? How? Here are two answers. "I would prefer not to see anyone suffer, not to do harm to anyone. But then I realize that the species is in danger..." "I thought I was playing a game. I didn't know it was the real thing. But...if I had known the battle was real, I would have done the same thing. We thought they wanted to kill us." The first words are Hitler's, the second Ender's. But the idea is the same, an appeal to good intentions. To save our people, we had to eliminate the threat presented by the existence of the stranger.
And that's a valid argument, if you're still a child and no one has ever told you what the road to Hell is paved with. It's a matter of historical record that Hitler honestly believed that the people he defined as human were in terrible danger from "inferior races." He did not merely use the threat to Nordic racial purity to become Fuhrer. Rather, he became Fuhrer because there was simply no other way to institute the sweeping racial programs his beliefs required. As Waite writes in The Psychopathic God: "The horror of Hitler was this: he meant what he said, he lived by his ideals, he practiced what he preached." And this, precisely, is the horror of Ender the Xenocide.