One Piece is not a "scary" story, though it sure has it's moments.
THis doesn't exactly count as it's one of the movies (even though this is in the animà forum, I rarely count anything that's not from the manga as being "real" one piece), but watching Luffy do the Gomu Gomu no Ikebana on Gasparde in movie 4 was one hell of a shock. I never expected luffy to impale anyone like that (even if the candy syrup power caused him to survive).
In the "real" story, there were moments like when Kurobi dragged Sanji towards the bottom of the sea, attempting to crush his organs. Also, Bellamy really left a shock with me in hos introduction (more in the manga than in the animÃ, though). I mean hell, he kills a guy because he loses a card game! He stabs his hand, shoots him, hits him with a bottle, flings him out a two-story window, jumps out of ANOTHER window and steps on his head. What the SHIT, man…