Love is a facet of the power of darkness, but that's not all there is to it.

The moogles explain to Sidurgu that the power of darkness is fueled by love, and Rielle chews him out for using his duty as an excuse to indulge in his vengeance, but it seems to me more that Sidurgu was being dishonest with himself than running entirely on hate and rage. He stated his duty was why he fought, and beneath that was vengeance, but beneath that was his growing affection for Rielle he was denying to himself for fear of losing her. Thus, Sidurgu was unable to fight at full power because he was denying himself.

The power of darkness is, essentially, the power to wield your own heart as a weapon. The problem is that there is darkness in every heart ("We all have a little bit of void in us. But it's quite healthy.") and to use your heart as a weapon without losing yourself to your hidden desires you must know those hidden desires. In our case, it's freedom from Hydaelyn's yoke; in Sidurgu's case, it's that he wants to be loved and cared for, and wants to love and care for others in turn.

Dark Knights are neither guardians nor destroyers - they are both at the same time. Recognizing and accepting this is what gives you the power to use your heart as a weapon and wield the power of darkness to its fullest extent.