The dark fantasy genre isn't well-defined. In its current form it's mostly a phrase people started using after ASoIaF became popular, but previously it was used to describe a whole lot of things, including horror. Horror is easy to define. It's intended to scare or repulse the audience.

Generally speaking, the genre of a story is often hard to pin down, because people react to stories in different ways. For example:

What genre does The Witcher belong to?

It has dragons and elves and other typical fantasy stuff, and is often light-hearted, so it's fantasy.
It deals with dark themes like racism, torture and the consequences of war, so it's dark fantasy.
It has scary monsters, and repulsive descriptions of limbs getting chewed off and the MC turning into spongy monstrosity, so it's horror.

The bottom line is that it doesn't matter. Genre is just a tag people use to help describe what a story is like.