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Nightmare Fuel: With Big Shells and Wings
Trowzer disecting a koopa troopa alive in front of Dix just so he'll persuade him to stop asking questions about his personal business. The author even put up a warning at the beginning of the chapter.
Doopliss finding Darrik's severed jaw lying on the floor in his house.
The first time he starts Laughing Mad and drops his seemingly affable attitude, and then he promptly tells Mario that he's going to torture him psychologically and force Mario and Luigi to lose their sanity. Although he doesn't in the story, the mere Foreshadowing for the sequel makes it seem like he'll make good on his threat...
The second time is even worse. As Trowzer starts to brutally beat Larry to death, Trowzer devolves into a horrifying monster with an unquenchable lust for blood. At some point Trowzer started to abuse Larry in such a way that Larry wound up becoming a "bloody bloated sack of bones." Trowzer became so crazy in his feral state that he scared every single soldier in his army and even made a few of them break down and cry.