Mostly harmless character randomly does some horrific shit
1. Invader Zim (Dark Harvest) - While Zim swears to dominate earth, he's usually far too dim-witted and short-sighted to do any real damage and his plans are usually foiled before anything can happen.
In Dark Harvest however, Zim is terrified of getting his alien anatomy discovered by the hospital, so he full-on starts stripping people of their vital organs and cramming them down his mouth. Dip then comes to school to see people writhing with their bodies shutting down and a morbidly obese Zim gagging out people's intestines and slurping them back up like spaghetti.
2. Mugman (Piano Lesson) - Mugman did get some development as a more capable, assertive leader in Season 2 but in Piano Lesson he full-on kills his piano instructor. After Cuphead gets chosen instead of him for the big show, he devises to cut off his brother's hands and drop a piano on him.
However, Ludwig steals both cups' show at the end and takes the falling piano instead. The radio delivers it like he has definitely indeed been crushed to death and the brothers just shrug it off like nothing.