You can say that the rise of the Nazis was due to flaws in the Weimar political system and weaknesses in social institutions at that time, but you can also make sure people know the names of Eichmann and Heydrich and similar.
What makes this a delicate problem for the democratic institutions is that 50+% of the voters voted in Bush for a second term even in the light of the paying for prisoners and tales of torture and worse, from survivors like the three British citizens just months before the election who went on to make a movie about it.
If we still want to make sure something similar doesn't happen again, it is necessary to have a juridical institution specifically to protect human rights with the power to veto these decisions, if neither the military institutions or the democratic populace will. Otherwise history has a tendency to repeat itself.