Finally, I got the Dark Ending on my 5th playthrough. It’s a troublesome-as-fuck ending that’s not worth it as it branches off in Chapter 13 and lasts 3 chapters. To open up the branch to the Dark route, you have to have obtained the True ending first, and then pick enough bad conversation options. There’s an invisible karma meter going on, and the easiest way to guarantee the opening of the Dark route is by repeating Chapters 1-3 because they have the most conversation options, and repeatedly act like a jackass. The branch will eventually appear at Chapter 13.
Spoilers below, for people who don’t want to bother with the Dark route.