Many people are afraid of the dark. By that they mostly speak of the darkness outside on a moonless night when there is no sun or lamps to light up the world for them.
I think that the dark places in a person are much more terrifying since you never know how tainted a person are or how dark the world is for them. Not knowing can lead to horrible things. It can lead to Grindelwald or Voldemort but it can also lead to the girl in third year who jumped from the astronomytower or the 17 year old boy who cut too deep in the bathroom two years ago.
Yes, the dark places in people are much scarier than any kind of darkness there is.