It's a common misconception! People assume the same things about pit bulls...
Ii find it hilarious because the only one I've known came running up to me barking the first time I saw him but then just flipped onto his back for belly rubs. Very vicious guard dog indeed.
They probably could be violent if someone broke into the house of its owner or something. But I doubt they would be for no reason unless not trained properly. Since usually dogs are disobedient and dangerous because they weren't trained and taken care of well.
Dobermans are actually the smartest and sweetest dogs ever, but in comics, books, movies, etc... they always show them as vicious and evil.
It's like racism for dogs!