Dark knights began as an Ishgardian knight discarding his oaths to stand up against the corrupt elements of the Holy See and vowing to use the power of the abyss to fight against it. However, despite the edgy themes, the class lore points out (spoilers for the 50-60 job quests):
... that one cannot properly defend with nothing but anger and hate. A true dark knight must also balance the darkness with love and the true desire to protect, or the only thing they face down the path is isolation and self-destruction. It is, essentially, a deconstruction and subversion of the dark and brutal anti-hero archetype.