This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Education for Death
Along with Night on Bald Mountain and The Mad Doctor, this is easily one of Disney's most chilling and powerful works. The scariest bit is probably the ending, where the long lines of marching soldiers are replaced by white crosses marked with swastikas.
And the end monologue about how the once-sweet children of Germany are now brainwashed into brutal, heartless monsters who only do, say, and believe in what the Nazis do, say, and believe.
Especially how the Disney animators had done to view these things.
The simple fact that it was based on reality. Other Disney villains are scary, but at the end of the day, they're just cartoons. Evil witches, giant dragons, and mountain demons don't exist, so there's no reason to be afraid of them. Nazis? They were and are very much real.
The caricatures of Adolf Hitler, Hermann Göring, and Joseph Goebbels.