There's not much unique-ness when it comes to tank jobs, are there?

[Absimiliard]

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.

I would encourage you to go research both of these things more thoroughly. FFIV explicitly states that the dark sword can and does negatively impact everyone that takes it up. Just because Cecil hadn't yet succumbed to its corruptive influence doesn't mean it wasn't having an impact - he says himself that it's changing him.

FFXIV's DRK lore, meanwhile, directly states that any dark knight that fails to properly contain their emotions can be overwhelmed by them to the point of being lost forever. It does indeed pose a great risk to your psyche. A dark knight in FFXIV is taking an immense personal risk each and every time they reach down into their emotions to call upon that power. Fray (the real one, not our aether-forged simulacrum thereof) was actually headed down that very path. Stronger emotions equate to a more powerful dark knight, but becoming too immersed in them exacts a terrible price. They temper their wrath with love for a reason. The process by which DRK functions is described the same way both in-game and in officially sanctioned external materials. This is what they are. What their power is drawn from does not detract from the fact that their purpose is to protect those they wish without considerations as to station or institution.