This is a fairly simple, straightforward question that I hope I can get an answer to. Where is the line of separation between an effectively dark storytelling and trying too hard to be edgy with risque or offensive subjects? There should be a clear distinction between nihilism for the purpose of story and nihilism for the sake of coming off as cool, and I’d love to know.
Yes, there is a clear distinction. Everything you put into your story should tie in with your plot. If you are adding stuff merely for shock value then it doesn't belong. If you want your story to be edgy, dark, morose, mature, etc., that's fine, and doable, as long as it moves your story forward.
The best way to determine whether or not you have crossed the line into writing just to be "cool" is to ask yourself these questions every time you come up with a questionable scene/event.
How does this move my story forward? --Every scene should have an effect on the plot. It might speed it up, slow it down or change the direction it's moving. There maybe a learning experience needed or item discovered that will be used later. If you can take a scene/event out of your story without changing the story, then it doesn't belong.
How does this scene effect my characters? --If a character doesn't care about someone dying, then your readers won't either, no matter how gruesome or tragic it is. If your character does care, you still need to show how it effects her motivation, and thus the plot.
As far as the interesting terms you've used, "2edgy" sounds like a great tool for a plot or plot twist, as long as it doesn't go off on a tangent. And "edgelord" could lend itself to some fun character development that could play into your story.