To be fair, there isn't really a reason that Dark Knights couldn't be portrayed as fearsome figures, driven by ambition and their own personal interpretations of right and wrong, who mete out, "Justice," indiscriminately and with brutal, sometimes cruel efficiency. Sure, a Dark Knight killing a bunch of bandits makes sense and works just fine with what we have now. But, what about a dark knight who kills a homeless, destitute person just trying to feed their children by stealing from a vendor? When confronted by the fact the man/woman they just killed had hungry mouths to feed, their response can be cold. "They'll be better off in an Orphanage than suffering under a failed parent."

Seems pretty dark and borderline evil, no? I mean, technically, they're still bringing, "Justice," by stopping a Thief, but the lack of empathy or any modicum of mercy is pretty dark.