|“||Oh, monster of so little size... is that a parasprite before my eyes?||„|
|~ Zecora in My Little Pony: Friendship Is Magic.|
Cute is Evil happens because people (both In-Universe and in Real Life) have a habit of sliding to the cynical end of the Sliding Scale of Idealism vs. Cynicism. It is also related to the phenomenon of people losing the ability to think critically around cute things. In places where critical thought is vital, cuteness can be the cover for a Person of Mass Destruction.
Parents, older children, and teenagers tend to never trust the adorable. "Real men" especially hate cute, as it is contrary to more manly forms of expression. Television companies who try to keep girls liking cute as long as possible are often portrayed as Corrupt Corporate Executives.
Cute is often evil in darker and edgier works. In extreme cases, cuteness is a sign of someone being secretly an Eldritch Abomination.
A frequent component of Subverted Innocence tropes such as Cute and Psycho, Corruption by a Minor, Killer Rabbit, Face of an Angel, Mind of a Demon, and Crapsaccharine World.
Very often the core of morally questionable, if not outright malevolent, children types such as the Creepy Child, Kids Are Cruel, Deliberately Cute Child, Spoiled Brat, Evil Orphan, Little Miss Con Artist and especially the Enfant Terrible, who exists mostly to enforce this trope.
If they succeed at causing death and destruction, this trope will likely overlap with Grotesque Cute or end up showing that Evil Makes You Monstrous.
Contrast Beauty Equals Goodness. Compare Beauty is Bad, a milder form. Also see Killer Teddy Bear.
Compare and contrast Adorable Abomination, which may or may not be evil. In some cases it can be played as a near-inversion of this trope, by creating a cute, friendly version of an Always Chaotic Evil creature.