The Dark Arts only look dark from the outside. Reality is more complicated, predictably less black and white. Also, the main defense against dark arts IS dark arts. Also, there's little virtue in not using abilities you don't have on moral grounds. Finally, we all have a lot of psychology here, but acting out behaviors you don't understand will help you to understand them.

Very seriously, those who relinquish the known dark arts will invent their own path to the abyss, a path without the protective guard-rails and warning signs worked out by billions of predecessors. We're much better in this crowd at overcoming the surface manifestations of self-deception promoting processes than we are at resisting self-deception. We end up self-deceiving in unusual ways, but predictable ways for someone who has met enough rationalists.