In the other post about well hidden twists, the movie Serenity came up, which reminded of the other Serenity with Anne Hathaway and Matthew McConaughey. The twist was so bad that it managed to trivialize the child abuse. In hindsight, it's kind of surprising the movie just disappeared, instead of joining the pantheon of notoriously awful movies.
What other movies with aspirations to be "serious" had wretched twists that reduced them to complete self-mockery? Malignant doesn't count because its twist was intentionally meant to give it a Drag Me to Hell comedic feel.