The song isn't actually about Adolf Hitler
at all, nor is it about any real figure.
Like the rest of Who's Next, Behind Blue Eyes was originally written for Lifehouse. When the rock opera portion of the project fell through, the songs were placed, without context, on the new album.
According to the liner notes of Pete Townshend's Scooped, it is the villain's song in Lifehouse. The villain in the story feels he has to put forward a false face, and the song is about that, and how it drives his behavior. The speaker, due to his position, cannot express any emotion, even though he is as human as anyone else. He also hints that he's not really evil, merely misunderstood:
But my dreams
They aren't as empty
As my conscience seems to be.
First of all, the villain's conscience isn't empty, it merely seems that way to an observer - in reality, he has a larger plan that he's trying to follow.
Finally, he tells the listener that in spite of his pretense of impassivity, he's as vulnerable as anyone else, and has to rely on other people. I assume (without knowing the minds of the band) that this was to give the antagonist more sympathetic and lend an air of moral ambiguity like the villains in similar dystopian science fiction works like Brave New World and Farenheit 451.