Homo homini lupus est.Many humans-- not just a Defector from Decadence or Misanthrope Supreme-- have observed that humans are perfectly capable of treating each other rottenly. Works featuring this trope naturally lean hard on the cynical end of the Sliding Scale of Idealism vs. Cynicism. Similar to Humans Are the Real Monsters, but there the observers of human misbehavior are separate from humanity in a significant fashion (usually aliens but sometimes also robots, supernatural entities, or a Human Subspecies). Compare Humans Are Flawed. Contrast Humans Are Good. No Real Life Examples, Please!! It'll just end in tears.
-- atrributed to Plautus
Hello, Unknown Troper. You'll need to get known to lend a hand here.