As i have always seen it,(& later proven correct by those damn moogles) A Dark Knight is all about sacrifice & conviction. Love is what makes a Dark Knight do what others fear to do, love is what guides us to the darker, more brutal, & sometimes questionable power we use to protect others.
Few Dark Knights will grow old, fewer still will ever die peacefully in bed surrounded family & friends sharing stories of glory & comradery. However glory was never the reason why we do what we do when it needed to be done. There is no guild for Dark Knights, no order or brotherhood, just the vow, the vow that there is someone who needs to be protected no matter the cost to our reputation, body, or life.
It never means we don't feel the guilt of our actions, if anything we carry the burden willingly, after all, it weighs as it should, but to keep those we care for safe, reaching through the dark flames is a thing we do smiling, burning the whole way, knowing the guilt will follow, holding the burden as we should.
I'll never be able to see a Death Knight who gains power from another's suffering is anything like a Dark Knight who sacrifices themselves for power to save their cherished ones the same.
After all, one needs only remember: "Serve...Save...Slave...Slay..." Fray Myste.