A bad guy commits a minor but key evil act, setting up his imminent death.
So, the Big Bad has kicked many dogs, Jumped Off The Slippery Slope, crossed the Moral Event Horizon a few times over, and has truly become a Complete Monster. And yet, this character is still given remarkable leeway by the universe and The Hero, who may be convinced that even a Complete Monster is redeemable, although the audience can see that it will never happen. And then, this character does something simple and minor. He decides to Kick the Dog just one more time, choosing a target that The Hero in some way considers sacrosanct, a target that triggers The Hero's Berserk Button. Suddenly, all bets are off for The Hero, who decides that this Big Bad must be stopped at all costs. It could be as atrocious as threatening the family of The Hero, or as innocuous as putting down The Hero's choice of clothing. Whatever it may be, the balance shifts, and it becomes clear that the Big Bad Complete Monster is about to die or otherwise be neutralised as a result. Occasionally, The Hero isn't the one who actually dispatches the villain. However, The Hero will still be quite happy with the result. Compare with the Self-Disposing Villain, especially when Hoist by His Own Petard. Contrast with Karmic Death, where it is a major, Moral Event Horizon type act that brings about the villain's death. If the villain's Last Dog is The Hero, may take the form of a Backstab Backfire. As this is typically a Death Trope, expect spoilers.
Examples:Anime and Manga: