It is unfortunate that the arcane discipline known as "Dark Magic" has acquired such a pejorative name in the common parlance, as it tends to relegate the practitioners into that class of sorcerer slanderously known as "evil wizards." To counteract such dangerous libels, it is handy to keep in mind the following pretexts:
PRETEXT THE FIRST: Insofar as it negates, drains, and preys upon the magicka and power of other mages, knowledge of Dark Magic is a necessary safeguard that enables the reining in of rogue sorcerers.
PRETEXT THE SECOND: Insomuch as it replicates some of the deleterious effects of spells cast by inimical Daedra, knowledge of Dark Magic is a useful tool for learning how to counter said effects.
PRETEXT THE THIRD: Whereas its application is regarded by the ignorant as frightful and loathsome, use of Dark Magic to inflict condign punishment upon transgressors is a deterrent to crime and therefore a social good.
That should silence the critics.