I'd recommend OneShot. It's an adventure game about making irreversible choices and the dilemma of whether to save a world or not. The message is clearer in the original freeware version, where you are warned not to close the game while playing. If you ignore it, the next time you open up the game, it plays out somewhat like what happens in Undertale's Genocide ending. The remake expands on the story and the gameplay, and is much more lenient on the player. You can find the original here and the paid remake here.