I love digging into language and this question is a little gem — what word most fiercely opposes integrity? To me, integrity is more than honesty; it’s coherence between values, words, and actions. So the opposite has to attack that whole package: the moral compass, reliability, and ethical consistency. That pushes me toward 'corruption' as the strongest single-word antonym in many contexts. It carries the sense of moral decay, bribery, systemic rot, and a breach of principle that’s both personal and institutional.
That said, English is rich and context matters. If I’m talking about a person who betrays a friend or trust in a dramatic, personal way, 'perfidiousness' or 'treachery' hits harder emotionally — it feels intimate and poisonous. For hypocrisy or false virtue, 'duplicity' or 'insincerity' is sharper. For legal or civic breakdown, 'venality' and 'moral turpitude' bring a more technical, damning flavor.
So I usually pick 'corruption' as the umbrella opposite of integrity because it implies a breakdown of moral structure across the board, whether in a single person who’s sold out their principles or in an institution that’s rotted from the inside. Still, I love how English lets you fine-tune the sting: sometimes you want 'perfidiousness' for betrayal, other times 'duplicity' for two-faced deception. Language is delightfully nuanced, and choosing the right antonym feels a bit like picking the exact color to make a scene pop — satisfying every time.