I think expressing our darker sides is really important. If we don't acknowledge that it's there, then it can turn very bad...like many things have in the world when people repress things.
All I can say is that I'm glad Clive Barker got his dark side out on paper...god help us if he had acted out his disturbed fantasies onto real people.
Anything that is disturbing to look at or read is challenging to the viewer, and I think that's what makes art so interesting. That's not to say that things that are easy on the eye are not equally as valid, just that they have a different effect on us, and we don't have to try and understand them so much.