I think gore is different than darkness, though. Darkness deals with the theme of the story - gore is just blood and deaths. It's what you do with the blood and deaths that makes it dark. The meaning of the deaths make it dark. Otherwise, it's just a gimmick to get your attention.
And I think this is a really important distinction to make. The darkness of THG - and BATTLE ROYALE as well though I've never read it - comes from the themes, not the execution (no pun intended, I swear). And - bit of a derail here, I'm sorry, but it's sticking to the same theme - that's why the Joker in The Dark Knight will always be scarier than the slasher dude from Scream. It's not about blood, and guts, and gore, it's about what all that stands for that makes it so terrifying. And that's important in literature. There's been a lot of mention of rape in YA novels on this thread, or that kind of thing. And while that's horrible and disgusting to imagine, it's not dark. What's dark is the idea behind it, and the harm it leaves behind, psychologically.
Thoughts and intents, will always be scarier than actions, in my opinion. Being killed, pa, everyone dies in the end. But knowing I'm about to be killed, and knowing that there's nothing I can do about it? Terrifying.