The brutality his troops displayed in Eastern Europe turned every hand against him, and interfered with his immediate goal of knocking over the USSR. He was so hell-bent on doing things his way, and living up to his every warped agenda that he alienated many people who would have been natural allies against Stalin, such as the Poles, Ukranians & Balts, and even ethnic Russians who detested their tyrannical dictator. Without even bothering much about it, the Germans, even late in the war were able to form a decent-sized unit of Russians under a captured Soviet general named Vlasov. A more public-relations-savy operation in the beginning of the war might have better exploited these feelings and gathered allies. If he was THAT set on disposing of the slavs, why not wait until he has knocked out the largest slavic nation and eliminated its army as a factor? And of course, there are the Jews. Some Holocaust deniers have advanced the argument that it would be too stupid to be believed that a country in the middle of a desperate war for survival against foes that outnumbered them badly and had far more materiel and resources would waste time building concentration camps and rounding up and guarding and executing people who posed no military or immediate threat. All the troops & transportation wasted on the Holocaust-related endeavors could have made a significant difference in the fighting. Maybe enough to push the Germans those last few miles to Moscow or prevent the Sixth Army from getting surrounded at Stalingrad...

But if Hitler didn't do those things, he wouldn't have been Hitler. He invaded Russia because wiping out the Slavs was a major part of his goal. If he wasn't a racist subscriber to fringe sciences, he wouldn't have had any reason for starting the fight in the first place.

Likewise Napoleon. Sure, if he hadn't invaded Russia, he might have lasted much longer, but without the arrogance and ambition that drove him to pick a fight with his erstwhile ally, he would not have come as far as he had.

Or Julius Caesar. People wonder what more he could have accomplished if he had not been assassinated, but it was his "accomplishments" that drove people to assassinate him. If he had not been the kind of guy you want to assassinate, he would not have gone around conquering places to aggrandize himself, or had himself made dictator for life.