It's all about the True Demon ending, though you did forget the Demon Ending, where you're so indecisive the final boss leaves the Vortex World out of disgust, and you rule it as a king until the next Conception cycle.

To me, Nocturne, and MegaTen in general, has always been about exploring the philosophies that govern the world than actual character. It's more about the lore and ideology than people. I always found it amusing that the Darwinian Yosuga faction was the friendliest to you up until Chiaki took them over, whereas Mutsubi (the closest thing Nocturne has to Neutral) are the biggest jerks. With all that said, Lucifer's path and Amala Network are so well done, it's hard not to gravitate towards it. Not to mention it kind of cements the Demi-Fiend as the biggest badass in the series.