Darkness isn't intrinsically good or evil. When discussing their actions in Mysidia, Cecil tells Kain that he took up the dark sword to protect the people of Baron, rather than to pillage and kill innocents. His doubts are what drive him to eventually turn his back on Baron, still as a Dark Knight.

Likewise, being a Dark Knight in FFXIV doesn't rob you of your humanity. It's the exact opposite. It's about being true to yourself. It's about freeing yourself from the shackles and expectations that others place on you. The warrior of light. Our weapon of light.

Listen to my voice. Listen to your heartbeat. Listen for the other...

On a throne of bone she waits...
Serve... Save... Slave... Slay...
They come to entreat her judgment...
Serve... Save... Slave... Slay...
One by one, in solemn procession...

A chorus of voices cries out for a hero, and she comes.
She smiles. She nods. And she remains silent...
But she too has a voice...
I will be heard...

The crux of the entire Dark Knight storyline is about finding a voice for your voiceless player character. By harnessing your emotions and giving in to the abyss, you find liberation.

Let's make the most of this. No style, no guile, just chaos. Mark your limits, then push yourself beyond. Let the darkness guide you and set you free.

...You stand at the precipice, but do not fear the fall. Cast yourself into the abyss, and you shall soar above, free at last.
Only when you have renounced everything are you free to do anything. When we meet again, you will give us your answer.

A heart bleeds, a man weeps, a soul burns. Thence comes the darkness, to consume...
Yet even in the depths, the flame endures...
Submit to the flame and harness the abyss...

The point is, your master was absolutely right when he told you to “submit to the flame.” You just never figured out what he meant by it.

It all seems so simple in retrospect...
The moment I stopped trying to find the answer, there it was. All that anger, all that rage...born of a fervent desire to protect that which we hold most dear.

Being a Dark Knight is about acting with humanity, not servitude to abstract ideals. What laypersons call the "darkness" is actually a manifestation of your inner voice and individuality. It's about being true to your feelings and protecting the people most important to you.

I wanted this, once. More than anything. A chance to speak and be heard...
But neither of us needs it anymore. And if we were to look deep within ourselves, we would realize that we never did.
We are the stories we tell ourselves. The brave hero, the tortured soul, the altruist, the pragmatist. They will tell you who they see, but you and you alone know who you are.

I'll say this again: Where others are bound by their Oaths to serve corrupt institutions and tyrants, Dark Knights are free to aid and protect those that they care about. Not because they're asked to, obliged to, or required to, but because they want to. Find yourself, and free yourself.