This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Pages of Harmony
Just about everything that Twilight does is this.
The various death sequences. To summarize, each of them is quite graphic and violent, and all are described in extensive detail.
Its not just the way Twilight kills her friends, it's the fact that each method to extract each element is ironically tied to the pony's personality. And then there's the realization near the end that in order to gain the final element she would need to completely destroy herself.