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Nightmare Fuel: White Night
Some ghouls attack and kill two sixteen-year-old Wardens-in-training (and is implied to have raped one of them while doing so). Harry proceeds toroast one alive, then throw the other one into a pit, melt the sand around it into glass, then pour orange juice on it head so it'll be eaten alive by ants. Carlos eventually convinces him to have mercy on Ghoul #2 and shoot it in the head, but still... wow.
Hell, any of Harry's dark moments are pretty damn frightening. Remember when he blew up a half-dozen storefronts cause he was frustrated? Or how he showed Molly that she wasn't ready to help him by flinging a miniature sun at her and challenging her to block it? Good Is Not Nice, indeed.
'Flinging'? He let it slowly drift towards her head, just to give her more chances to try and fail. While taunting her for not being able to stop it.