Hmmm...interesting topic.
I don't think adolescents are all that different from adults when it comes to their aesthetic instincts, except that they are not developed cognitively to the point that they are as receptive to the subtler sorts of symbolic or thematic connections as adults would be. Therefore the content (the connections) are by nature more physical, more obvious, and more extreme.
Even so, most of my favorite adult literature, which can be incredibly subtle, deals with the same sorts of things. Hamlet touches on incest, adultery, and murder, and concludes with four characters dead on the stage. Flannery O'Connor's "A Good Man Is Hard to Find" narrates the killing of an entire family. Another story ends with a child committing suicide. Another ends with a retarded girl being abandoned at a diner. And O'Connor was a devout Catholic, who was a devoted critic of moral relativism.
It's an old debate. And it always ends with the following truth: literature, even great literature (and perhaps especially great literature) cannot be regarded as offering humanity any sort of guide to moral behavior. If we use the great literary characters as moral guides, we will end up as vain and selfish monsters. It is instead a device that allows us insight into the range of human experience that we would otherwise have a hard time accessing and appreciating. It allows us to know what humanity is, for better or worse.
But I also understand where the lady is coming from with her article, and I'm not unsympathetic to the view that literature has the capacity to mold our values. Indeed, I believe it does exactly that for kids who lack moral direction from their parents.
And therefore it looks like to me that we should hold off on allowing all kids to read all types of literature. Instead, here is what we will do: For those that come from homes with solid moral instincts, they will be allowed to read "dark" things. They have the moral framework in place that will allow them to put the "darkness" in its proper context. For those kids who lack moral direction from home, they will be directed to books that deal with more mainstream content, and which might serve to reinforce those values that will allow them to be contributing members of society.
I see no other way to go.