I also believe that the derplander is just a placeholder for "all of us". In the CGIs he is shown witnessing the "death" of Nanamo (like we did as PCs), as well as moving to Ishgard and switching jobs to impale dragons with spears as stiff as ice.
The Dark Derplander is, in my opinion, what our PCs CAN become if they keep going on their path - getting more and more powerful, annihilating everything that stands in their way (including dragons which, per se, were not evil at all).
As seen in FF3, the "Warriors of Darkness" are not inherently evil either - I see them as "anti-hero" stereotypes rather than antagonists.
Remember that, in the FF lore, there can be TOO MUCH light - and that is as bad as having Darkness engulfing everything. Balance must be maintained at all costs, and since Light has been winning for years now, it's time for Darkness to bolster up its strength and even things up.
Like the Light Emperor in FF2 and the Archbishop in HW, we can clearly see that Light can be twisted to "evil" purposes anyway. Hell, the Archbishop sees us as darkness incarnate just before dying - we might have been playing the role of Warriors of Darkness already without knowing it. What if Elidibus planned this? After all, the Archbishop kinda betrayed the Ascians so he had to be purged - his "Holy Light" had to be snuffed out. It's only natural that, if you help defeating the "Light", you are automatically a "Force of Darkness" - no matter if the Light you defeated was evil in the first place.
One thing that stroke me as odd the first time I read it was the line that King Thordan says on screen when he is defeated. It's something along the lines of:
"This is the power of Light..?"
Now, that can be interpreted in two ways. Either he is amazed by how pro you are at wielding the Power of Light, or he is doubtful that you are wielding Light at all. Considering how he sees you after that, I think the latter.
We in the west tend to believe that "light = good" and "dark = evil", but this should not be taken for granted.