Choose Goose became evil because he drank the "ichor of Coconteppi," a magical substance that transformed him into an evil entity. This event happened in the Wizard City storyline from the Adventure Time universe, specifically in the Fionna and Cake series. The "ichor of Coconteppi" is linked to dark magic, and after consuming it, Choose Goose's character turned from a rhyming, somewhat quirky goose into an evil being.