I've always wondered where this ideology came from. The real life origin of Dark Knights, the legendary black knights of medieval Europe, also known now as Knights-errant, were knights who wandered the land defending the people or seeking adventure to prove their chivalric worth or simply to earn the affections of a beloved.
In the series as a whole, named Dark Knights have always straddled the line of heroism and villainy. The very first, Leon (FFII), was a villain for most of his game, while Cecil Harvey (FFIV) redeemed himself. Zeid (FFXI) and Sidurgu (FFXIV) are both flat out heroes in their own right, who, while ruthless, get things done.
The Job doesn't really imply any mechanics either, while yes, they traditionally have cast from HP, Dark Knights in numerous installments use magic outright or forego this mechanic. Additionally, Dark Knights, while typically very offensive, usually buffer their self-consuming ways with high health, defenses, and vampiric attacks, traits we see in FFXIV. Really, Dark Knights here suffer more from oddities in their balancing than their lore or how they feel to play.