How did Napoleon and Hitler manipulate the democratic system to win Public support for their military campaigns?

What happened to the Democratic process in Germany that led to Adolf Hitler being named Fuhrer?

Despite Adolf Hitler promoting a Fascist State and Joseph Stalin promoting a Communist State - two oppositional forms of government - explain how both leaders created mass genocide to their own citizens.

How did Hitler and Stalin strip it's citizens of their legal rights and use propaganda to convince the people to support their policies?

Are there any Modern examples between the years 1970-2020 in the world where a recent Modern Political Leader was democratically elected by their Countrymen and then tried to set themselves up as a Dictator?