To say that they voted in Hitler is grossly oversimplifying the issue. He was appointed--not elected--prime minister first.
In addition to political campaigning, the NSDAP engaged in paramilitary violence and the spread of anti-communist propaganda in the days preceding the election. On election day, 6 March 1933, the NSDAP's share of the vote increased to 43.9 per cent, and the party acquired the largest number of seats in parliament. Hitler's party failed to secure an absolute majority, necessitating another coalition with the DNVP.
To achieve full political control despite not having an absolute majority in parliament, Hitler's government brought the Ermächtigungsgesetz (Enabling Act) to a vote in the newly elected Reichstag. The act gave Hitler's cabinet full legislative powers for a period of four years and (with certain exceptions) allowed deviations from the constitution.[153] The bill required a two-thirds majority to pass. Leaving nothing to chance, the Nazis used the provisions of the Reichstag Fire Decree to keep several Social Democratic deputies from attending; the Communists had already been banned.
So after Hitler had twisted Hindenburg's arm into making the Reichstag Fire Decree, they physically barred the opposing voters from entering the voting chamber and voting down the last barrier. But yeah, it was all democratic.
I'm interested to hear what you think the U.S. voters should have done if none of Bush, Gore, Kerry, Obama, McCain, or Romney were worth our votes (not that I really disagree with you on that point).