Neither Napoleon nor Hitler were ever elected as heads of state. Hitler was eventually appointed Chancellor, but that is the head of government. The head of state is the President. Hitler was able to manoeuvre into that position using his personal army to murder opposition. Napoleon had an old fashioned military coup in 1799, then attempted to legitimise it with a falsified plebiscite in the following year.

In both cases, the issue was the murder, not the democracy. It is important that we not blame democracy for the actions of evil men.

Napoleon 3 was elected president in 1848 and became emperor in 1852, you are mixing up with the first Napoleon.

And then the head of state in the Weimar republic really was the chancellor, that's why Hitler could dismantle the republic.